yeah wasn't that supposed to be this month? I bought Portal 2 like 3 hours after finishing Portal. Clash, if you haven't played Portal 2 yet, definitely give it a go. An absolute gem. I haven't tried the co-op mode yet (just finished the single player), but I've heard great things about that too.
Yep, soon after it came out they said the dlc would be out in the summer. Then that turned into mid-September and now here we are...
But yes, playing the co-op is an absolute must. I played it split-screen with a friend of mine and it was easily one of the best vid-game experiences I've had. I'd imagine playing online co-op would be too frustrating, but not impossible.
There are actually quite a lot of really stellar custom maps that you can dl for portal and portal 2.