Edit: It also comes with its own sdk- cool.
I looked at the Alien Swarm SDK. They gave us example maps for Hammer, yay! This is a very good thing, as it looks like there's a bunch of new stuff with the engine for Alien Swarm. Also, the maps are big. And the perspective makes things challenging. Should be good!
Just finished the campaign with flame. Aside from the inconsistent difficulty curve, Its incredibly fun.
They should make a L4D version of this.
Oh, and it desperately needs more maps!