Steam is awesome these days! I was one of those who bought HL2 on its day of release and then spent a whole night trying to activate it while Steams servers went into Meltdown. Also used to crash quite a bit, kick you from servers and all sorts. Past couple of years though it seems to have matured nicely and despite the bashing it got when it first came out, seems a damnsite more tame than some of the DRM crap we are expected to swallow these days. Saying that, I wouldnt buy a game over Steam (without considerble saving) that I could get retail without the Steam requirement, as it sucks not being able to sell/swap something your bored with.
Zombie Panic is insane!