this is a fun game with minor glitches. i too like zombies. the difficulty is good the way it continuously gets harder. despite what debacon said, barricades are needed in the harder levels i don't care how good you are.
the only real glitch i found was when i tried to use the grenade launcher. it stop reloading midway through, then i was stuck because there is no way to change weapons in mid-reload. i think the problem occurred because i clicked to try and fire when i was reloading it.
i didnt like how expensive the merc's were, it cost way to much for a decent one. but then i found out about the money issue where i can still retain all my money when i die and keep my ammo and merc's. im not complaining about that though i wouldnt have been able to beat the game without that.
all in all, really good game, and you really know your weapons.