Started playing with a collage friend yesterday. The coop is actually really good. I was told that it was better then the single player but i wouldn't know since i haven't finished either of them. I am at chapter 7 and god this game is good. Funny, beautiful, great puzzles great "story". For anyone that has not played Portal yet i strongly sugest you start with Portal 1. Its a max 2h game. You just have to think through the puzzles first (if you finish the game you can finish it again in less then an hour, my personal record is 45 minutes :P).
The coop has 4 portals so its a little more complex and thats why its more fun. Also seeing your partener beeing smashed to bits is a lot of fun too:P. Will post more as soon as i play more :).
Also i am still playing Starcraft 2 (had a 3 mounths break from that and i am horible now, was mid diamond but now i am plat again :() and League of Legends. If anyone else plays do tell^^.