This one's really cool, I didn't really have time to get in to it, but at first glance, it looks pretty slick.
Left 4 dead is finally done, and that's pretty cool. Co-op is where it's at. For me, when I play a single player game, I just can't get in to it. Throw a few friends in there and then all of a sudden the pressure's on. Gotta be there for my pals, and all that. It makes for a more rewarding experience.
It would have been cooler if each player had unique characteristics so you could play the game differently every time. Maybe have an RPG system in there. I have a feeling after you've played through each campaign a couple times, it'll probably get boring.
The developer has to ask the player, "What's your motivation for going through this same level again and again?"
Sure, the enemies may be in different places or whatnot, but to me, it seems rather pointless. Been there, done that, moving on.