Co-Op is very nice in a FPS game.
I gonna take Gears of War as an exemple, the single-player is nice, but in the most high difficulties you gonna die a lot and your "CPU partner" is pretty dummy, so, with a human playing as your partner is much easier, you gonna finish the game is less than half of the time you going to take playing alone, and the double of fun

I really hope that Crysis 2 have an Co-Op mode. Play friendly with friends is pretty cool.