Even though Killing Floor doesn't have a story line I still prefer it for co-op fun. This is incredibly easy, and the scripted way the special zombies pop out next to you is annoying unless you have the maps memorized.
The major con about this game is not related to the game, but the subhuman cess pool that is the online community. The game itself is enjoyable, albeit not much different from it's predecessor. If choosing between L4D and L4D2, buy this one.
Excellent, brutal zombie killing fun. Online is not as good as it should be except on modified dedicated servers which evens up the game and makes it much more enjoyable. Nothing beats chopping zombies up with a katana though!