I was thinking of first posting this in
this thread, but I ended up deviating so much from the original topic I thought I might as well post this by itself. I was originally going to comment on whether the demo was OP but ended up writing a little tip collection for how to play demo and how to counter demo.
So first a disclaimer of sorts: I'm not nearly the best demo on the server, and I'm sure Conscript, Zoopy, Snail, etc can also easily provide good tips/guides as well, but as I've been playing demo so much recently (in my demo phase, so to speak) I thought I'd say as much as I can about demo before I get caught up with another class and forget about it.
So here we go. Going to start with talking about the basics: how the one eyed guy goes out and kills stuff.
I don't know the exact numbers behind these weapons, but then again I don't believe numbers give the whole picture: its enough to know that X class dies in Y hits from this weapon in a normal game situation.
Nade Launcher:
The supposed primary weapon for the demo. I used to disregard this weapon completely and use stickies as my primary all the time, but that's really a huge mistake. The nade launcher covers a lot of the holes the sticky launcher has, and if you only limit yourself to the sticky launcher, you're simply denying yourself a third of your arsenal.
The greatest strength of the nade launcher is in two things: the great damage from the direct nades, and the ability for the nades to bounce and roll! Once you get good with arcing the nades into enemies, its really a formidable weapon. One direct hit from a nade can potentially kill a light class (scout, sniper, engy) with a single hit, and two hits generally kills anything not a heavy or soldier. Because of the great damage but necessary good aim, its pretty much a no brainer to always aim for large concentrations of enemies, or narrow corridors or people moving in straight lines.
Sticky Launcher:
The so-called winbombs. I'm not even sure how much damage they do exactly, but 2 kills light classes and 3 kill pretty much everything else not a heavy. There's just so many ways you can use it (that argubly it shouldn't be able to be used for). The most important things are: that you understand how stickies fly with different degrees of charge, and that you develop an ability to predict where exactly enemies will be moving. You can pretty much shoot, detonate, shoot, detonate when in combat and it works out fine (as of now). When moving up to the frontlines charging up a sticky and having it ready for long range targets is very useful to get at least that first shot off.
Bottle:
Don't forget you have this. Useful for spies, or pointblank combat in general. The demoman is only a tad slower than most light classes, and because its difficult to use stickies in point blank, its often very useful to finishing off something close. I probably pull this out a bit too often, but its useful for getting people to back off, and as you can still detonate stickies with this out, its useful for luring people into a false sense of security - I often switch to the bottle just as I detonate my last sticky, and often don't have to use it at all as the last sticky kills them.
Class Specific Strategies
Vs. Scout
Hello to potentially your worst nightmare. There's basically two situations here: either you're in the open, or you're in a closed in space. If you're a closed in space, do whatever you can to sticky off the path leading towards you to first secure your own safety. Scouts don't do much at long range, and once you keep him at long range (for now) you can figure how what next to do. Expect that he'll take an alternate route and wait for him there with more stickies. Out in the open, your only chance is to predict exactly where they are going to go. When I run into a scout, I'll shoot a sticky quite far in the direction they're going at the time, and then continue to shoot stickies where I think they're going to go. Against scouts that will always flank behind you in a straight line, this works perfectly and you won't have any problems. Against scouts that have a good aim, and will move in unpredictable patterns, you're screwed. Despite all of Santa's complaining that he tries to avoid me as much as he can, every time I run into him without prior preparation I'm dead before I can even do anything. If you're feeling lucky you can try to use the nade launcher, but generally if the situation is bad for you, throw down stickies to keep the scout away and retreat. Find a soldier or heavy to walk with.
One other strategy is to throw down stickies on one side of the scout, and pull out the good old bottle and rush the other side. This sometimes works when some scouts think that pointblank range is where the scatter gun does the most damage, and they literally run into you to try to kill you, and the bottle gets them pretty fast. The other situation is where the scouts will back off and try to circle around you, forget about the stickies you already laid down, and as you can still detonate with the bottle out you get them that way. This works surprisingly well against most scouts but if you ever play against unpredictable scouts don't expect it to work. Luckily there aren't many of those on the server. Playing against demos requires that you NEVER move in a straight line. if you play scout, as long as you never, EVER run in a straight line (even a circling flanking straight line) you shouldn't have problems against demos. even strafing left and right in place against me works as I will assume you're going to move and sticky your left and right sides, but won't actually sticky your current position.
Vs. Soldier
Take the higher ground whenever possible. Against soldiers like Conscript and thundeer expect a rocket jump so sticky their landing point and run like hell. Don't run in a straightline either when you're further than close-medium range - actively try to dodge rockets. Nades generally won't hit at medium or far range, and if the soldier is smart he's also run straight at you, which means your nades will roll past him and do nothing, as opposed to backing up and letting them expode on them. Stickies with a touch of charge work pretty well, as neither of you do that much accurate damage outside of close range. At close range, pray that he has bad aim. I believe in the fact that soldier has the advantage close range, and the only times I remember beating soldiers close range is with lucky direct nades, bad aim from the soldier, or I could hide behind map obstacles and splash him to death. Or with a trusty medic. Stickies work decently well if they're also moving in a straight line, but you get the same problem with unpredictable movement - the delay in detonation is long enough that if you don't have good aim you won't kill them in time. I generally try to avoid soldiers without some help or a medic on me.
Against demo, don't be afraid to rocket jump to close range and then rocket him from there. If he's using the nade launcher never back up or you're just letting the nades detonate on you. As long as you have decent aim and he doesn't see you coming you should have a good chance.
Vs. Pyro
If you're confident in your ability to hit a target running straight at you go ahead and use the nade launcher. Otherwise, I find that most often I run into a pyro at a range where he feels like rushing me has a chance, and I'll just sticky his most direct path to me, and then surprise surprise that's the path he takes. It turns out pyros that demos see coming aren't that big of a deal. Its the ones that suddenly surprise you at point blank that you'll have a problem with. If you use the nade launcher there you'll probably blow yourself up to, so I just look straight down, sticky, jump away and blow them off me, and then try to follow up with another sticky. I have a bad habit of jumping with my back turned and its gotten me killed a lot of times against a pyro with a BB, but you often can even run directly away from him, sticky the ground just in front of you, and detonate without looking back and the pyro will often be right on the sticky. If you have a medic or an alternate health source nearby you can often bottle him after the first sticky, which works surprisingly well - most pyros will just blindly rush into you. At farther ranges, as long as you never let him approach you won't have a problem.
As pyro, I know its tempting to bumrush a demo at close-medium range and think he won't get you, but more often than not he will, as long as he has space to back up. If he's in a corner, then yes you can go ahead and rush, but dont' be surprised if he smashes a crit bottle in your face. Remember that since the dropoff in flame damage is only at the very tip, you don't have to go into pointblank, just so long as your flame is burning him you're doing your full damage, and you won't risk getting bottled.
Vs. Demo
This is often just who gets luckier, depends on who uses what weapon, and where your relative position is. Stickies are often more reliable than nades, but if they get a lucky direct nade hit you lose. Not really any specific tips I can give other than to use standard combat tactics. Rushing demos is generally always a good idea as most demos don't know what to do now that their sticky trap isn't protecting them. I've run into a sticky trap on the ground, sticky jumped over it, and chased down the demo who now was spamming nades everywhere and not knowing what to do.
So first: sticky traps. please don't put them on the floor. on walls and edges of doorways are MUCH better, and the only times you should be putting them on the floor is when you WANT them to see them (you're at low health or are running away and just dont' want them to chase you) or there's simply no time. Second, if they bypass your sticky trap, dont' be afraid to use your stickies to fight them off - if you die, your trap's useless anyways, and if you survive you have a chance to resetting the trap. Third, I hate camping demos. You know, the kind that sticky a doorway and spam nades through it or another doorway for the rest of the game, or the kind that sticky a point and spam nades at the enemy team. While yes, its a valid tactic and useful in some situations, especially push maps like dustbowl or similar where the enemy is forced to come to you, it honestly does nothing than to piss people off. If you run into one of these, either find an alternate route or blow the stickies away with something. If worst comes to worst have someone take them, and then the rest of the team should run in there and punish the demo with whatever melee weapon is available. While doing this on dustbowl-esque maps is fine, doing this on say, granary or well is just not helpful. Yes you can sticky that wonderful middle point and spam nades at the other team for the rest of the round, but then you better not complain about sudden death or the round being dragged out because you're not pushing up. And if that's all you can do with demo, you might as well play engineer (gasp) or heavy. Learn how to fight in direct combat and gain ground instead of losing it. Or else thundeer will call you a brainless camper.
Vs. Heavy
Demo is a pretty decent counter to the heavy because of the demo's ability to deny areas, combined with the heavy's low mobility. If the heavy is spun up and in the open, the nade launcher works great and should be used. If the heavy is spun up and just around a corner, make sure you sticky the corner to make sure he can't push around the corner without taking heavy damage. In many situations using direct nades against the heavy is probably your best bet, and making sure you take every oppotunity to limit the heavy's movement space with stickies. Use the map to make sure youre never in the heavy's direct line of sight.
If you're playing and you run into a demo at any situation other than pointblank range AND the demo has nowhere to go, I would advise never spinning up and using the minigun. In my experience I've found demos don't know what to do when you run straight at them, and that's exactly what you should do with the shotgun. Of course, if the demo has any help its best to retreat, but bumrushing them with a shotgun works well unless they're smart enough to go with the nade launcher, which means you strafe and jump like and idiot while running. If you have a medic its best to tell them to not follow you cause they often get killed by a stray nade or sticky, as the optimal range for demos is just past you, which means your medic is the one that gets killed more often than not.
Vs. Engineer
Ah engineers. Just stay away and don't get crit wrenched. Not much I can say, he's got a shotgun and you have explosives. But with engineers...
vs. Sentrygun.
NOW we're talking. The one thing the demoman is supposed to be responsible for. Nothing annoys me more than having 2 or 3 demomen on a team that don't know how to take down guns.
-Learn how to corner! sentry guns built next to corners are just asking to be taken down. Just poke out, shoot a sticky, and then jump back in. If you don't have much time go ahead and keep jumping it and out, otherwise you want to wait a little bit so the gun rotates back to its normal position before you poke your head out again. 3 or 4 stickies in the gun's general area kills it pretty easily. If the engineer comes after you, go ahead and sticky and kill the engineer. Don't just ignore him: if you let him get close he'll crit wrench you so hard you'll be even more drunk

This doesn't just apply to stickies. In certain locations, using the nade launcher against guns is useful as you can bounce them and have them settle all around the sentry gun, damaging/killing the engineer and eventually the gun as well. There exist situations where the nade launcher is much better than stickies in taking down a gun.
When a nade directly hits a sentry gun, the explosion on contact does pretty hefty damage to the engineer as well, and with two direct hits you generally kill the engineer, and then you're free to destroy the unmanned gun however you want. If you use stickies against guns, the engineer generally abandons the gun long before you get down your stickies and detonate, and if your team doesn't roll in right behind you (you'd be surprised how frequently this happens) that gun will be up right away. Using the nade launcher can kill the engineer before he can react and often damage or kill other enemies standing around the sentry gun (often a medic healing from the dispenser, or other enemy players taking cover). I can't count the times I've blown my stickies prematurely when stickying a gun to try and take down all the enemy players standing around the gun but manage to do nothing, not even taking down the gun.
In the ctf_well demo I post here, I run into two sentry guns in their intel. I take one down with stickies, and the other I take down with the nade launcher because of the location of each gun. The first gun is out in the open, so I use a small charge with the sticky launcher to sticky it, and the second is under cover (cannot arc stickies onto it), far away (charging would need almost a full charge to bypass its cover), and the engineer is present. So I use the nade launcher to roll nades onto the gun, first killing the engineer with a lucky crit, and then continuing to roll nades onto the gun to take it down. Once you figure out where a gun is, there should always be a spot where you can use your arsenal to take it down - without uber in many cases.
Example that uses direct nades:
On Goldrush stage1 point 2, engies like building on the upper right ledge (offense POV) where it covers the major entrance. While good soldiers can take it down from the second floor walkway, its actually even easier to do as a demo. And not with stickies, as you need a pretty good charge and place them so they dont' bounce off the gun and fall down. Instead, stand on the second floor walkway (make sure no snipers are around first), and simply arc your nades towards the gun and it works out so that they directly contact the gun. Most of the time they kill the engineer first, and sometimes even the gun first, even with the engineer present. Once you learn that arc any gun there is useless.
Vs. Medic
Medics are actually rather dangeous if they know what they're doing. beware the bumrush bonesaw, or the lifesucking needles of doom. Make sure not to run in a straightline into their needles, or they'll heal faster than you can damage them. Stickies can be hard to land consistently due to their speed. However, if they start running away, you have them. You can either sticky jump to chase, or charge up a sticky as you give chase, and once you round the corner you can accurately a sticky to cover their escape route, detonate midair, and often finish them off.
This actually applies to all retreating classes. If someone starts running away and turns a corner, start charging up a sticky as you chase, when you round the corner, they're often running in a straightline back to health (you know where they will be going 99% of the time), and you can simply shoot that sticky right into their path and detonate. In the badwater demo there's actually two instances of this: once when I damage Snail the engineer twice with stickies, and as I chase him towards spawn I charge up a sticky, wait for him to run for the spawn gate, and then stickyshoot him. I try it again with Pablo the sniper, but cause he has suicidal urges he actually outsmartted me by not running for spawn, and trying to kill me instead - thus I waste time waiting him to go in a straightline for spawn, and he gets reinforcements quickly.
I can't emphasize enough how useful charging stickies can be: I've killed a pyro when I was standing at the front entrance to B on gravelpit and the pyro was standing in the B-C doorway due to accurately arcing a sticky across B and detonating in his face (Golden will remember this) You can often help out teammates a long ways away simply by having a sticky charged whenever possible. Remember: holding rightclick will cause your sticky to not automatically launch when it reaches full charge.
Once you use the sticky launcher itself enough, you get familiar with how much charge you need for different situations: it takes experience. Often a slight charge is needed for a lot of situations.