Heard about it, seemed interesting...just haven't given it a shot.
What game is the gameplay comparable to? (as far as controls, etc.)
Is the online multiplayer worth a crap?
It's got a single player component, but the game is really all about multi-player. It's a 4 (Left
4 Dead) player co-op styled like a horror movie, where the four players make their way to a a rendezvous point to escape a zombie apocalypse.
In addition to standard run-of-the-mill zombies, there are "special" infected with the ability to completely incapacitate one of the survivors, which means that teamwork is mandatory.
There's also a versus mode where one team of four controls the survivors and one team of four spawn as special infected.
It's a Valve game, built on an updated version of the same game engine as the Half-Life 2 chapters/Portal/Team Fortress 2, but the basic mechanics of the game mean that it doesn't play exactly like any of those.