The zombies are great. Varied but without being impossible. Spawning sucks balls though.
The weapons are great. Powerful, none of them are direct upgrades and are viable on all waves and difficulties, and have a really nice feel to them.
Perks are great, because they're like crack to level up.
Teamplay is surprisingly strong, and KF is one of the few multiplayer games where teamplay is strong.
Only stupid part is Tripwire. There is no reason why you should play any other game length beside Long, because many map achievements discredit Short or Medium lengths, and they really don't like custom content installed on your game.
So yeah, great game, definitly worth the $20, my second most played game in my library except TF2.