I found a very detailed thread on the Boomer class over on the official Steam forums. I have saved a copy below:
A Boomer GuideEnjoy!
The boomer is the centerpiece of any Infected team's offense, the skill of the boomer player can shape the outcome of many a versus match. Aside from tanks, witches, and the occasional horde... boomers are the only player-controlled means of generating an "opening" in the survivor's defenses. Normally (and I use this word lightly) a competent survivor team can quickly deal with the other 2 infected classes very quickly.
Why is he so important:
1) The blinding effect of your vomit makes it very difficult for blinded teammates to help each other out. Most of the time they have to resort to spamming their weapons at headshot level and spamming melee until the effect begins to wear off. The effect makes it almost impossible to save allied players at first unless they are very close to you.
2) The music that plays after a successful boomer attack on a players + screams of the charging infected also make it extremely difficult to hear what's going on around you and find out who needs help.
3) The obvious free infected swarm that comes with any successful attack.
4) The combination of the above 3 factors creates chaos and confusion among even the most coordinated survivors. The inability to see or hear teammates that well makes it quite a challenge to save them, and being surrounded by infected makes it very difficult to accurately shoot or run to their rescue.
So here are my tips for maximizing the usage of the boomer:
1) Don't attack until at least 2 of your teammates have spawned unless you think you can "get away" after vomitting on survivors (Not very likely)
2) Spawn IN FRONT of the survivors, or ABOVE them, spawning behind them is a dead giveaway for anyone with headphones and forces the boomer to follow them... boomers who spawn behind players are usually free kills unless there are very tight quarters... in which case why dont you just spawn in front of them anyway?
3) Tight corners and roofs are almost guaranteed "win" spots. 90% of teams will have no way to stop you if you camp the right corners, if they shoot you as they turn the corner they will be covered in vomit, if they run to melee you then you can vomit on them anyway... one more reason to spawn in front of them.
4) Don't risk your (boomer) life if the enemy is already fighting a horde. This may seem controversial but when a horde is coming the survivors are already hunkering down for the storm, they will be extra wary and even if you manage to blind them they are probably in a good defensive position where it wont matter. Furthermore adding a few infected to the horde doesn't really make much of a difference. On the other hand blinding + summoning infected after the survivors just finished a horde will wear their health and ammo thin and slow them down.
5) Boomers take fall damage, you can "suicide bomb" enemy players from tall roofs giving them very little warning.
6) Suicide > Vomit against better players. There is a slight delay before the vomit begins and your constant burping will give them plenty of warning, if given a chance to fall from a roof, charge around a corner while someone is firing, or simply run right into their midst and wait for someone to panic and shoot as long as one person gets "boomered" you will be doing your team a service. Many times I've seen novice boomers trying to vomit from rooftops or standing near corners and they went down without doing any good... suicide charges are much more reliable than a vomit attempt.
7) Learn the maps, there are a number of boomer "win spots" where it's almost impossible to avoid the boomer, if a spot doesn't work the first time it probably won't work the 5th time.
8) Consider re-blinding. Ok so you pulled off a successful vomit and lived, hunters and smokers are starting to dismantle the enemy team and the infected are making things difficult for them. Now you can stand back and ambush them at the next turn after they clear up and kill your teammates, or you can just wait about 12 seconds and charge in and suicide into them again. Blinding them again will keep them confused and blind for a combined 25+ seconds giving your teammates plenty of time to finish off their target and wipe the enemy team. Is Bill being wrapped up by the smoker but Zoey is blind firing trying to rescue him? Are 2 or more survivors being disabled by your teammates (smoker/hunter)? Does it just make SENSE to keep your enemy blind for another 10+ seconds? Then run into their blind gunfiring and slap them with right click and they will blow you up 99% of the time and buy your teammates an even bigger window to do their job.
1 - You make a distinctive noise and have a large body frame, rely on the "zombie vision" through surfaces to track your targets and don't expose yourself unless you plan on dying. I've seen so many boomers try from "peeking" over roofs and around corners. You have 50hp, one burst from the M16 will kill you before you can react at almost any range.
2 - Smokers... Hunters... you guys are practically useless on your own. Wait for the boomer to make you an opening unless there is a straggler survivor. That doesn't mean you can't run around and distract the survivors to give your boomer an easier opening to run in on.
3 - Smokers... Hunters... target those lovely NON-BLINDED survivors first. They are the ones coming to the rescue, the infected aren't targetting them as much and they aren't blind... they will gun down and free any teammates in a matter of seconds if you don't pounce/smoke their ♥♥♥. Likewise if 2 survivors are blind and 2 aren't... if 2 hunters go for the blinded ones first then you are just going to die a couple seconds later when their non-blind teammates melee you off or just gun you down. Does it make sense for a hunter to jump on a target being attacked by a smoker? No. Does it makes sense for a tank to go for a target being attacked by a hunter? No. Boomer vomit makes a target blind, near deaf, and unable to move because they are surrounded by infected (or will be soon), it's a great form of Crowd control and why would you cancel it out by just jumping on them? You will see your damage increase several fold when going for the non-blinded targets after a boomer assault.
4) Dying near the tank stuns it, which isn't very nice. Tanks spawns are probably one of the few times where it makes sense to come from behind while they are distracted. (courtesy of Faur)
5) Don't forget that your puke arcs. You can extend its range by aiming upwards. You can also use this fact to hit players you can't directly see by puking over objects/obstacles. (courtesy of Ydiss)