I gave Portal 2 co-op a try and really liked it .  It's a worthy continuation of Portal 2, both in game play and story.  There was enough new stuff to keep the game interesting, and not just rehashing what I had done in the single player.
 
One thing that I was amazed by was the first player I played with was really co-operative.  Honestly, in most "co-op" or team games, I find that few people work together.  But then again, you cannot get anything done AT ALL in Portal 2 co-op if you don't work together.
 
That being said, the second and third person I attempted to play co-op with had used some cheats to add weapons to the game and immediately attacked me, not only making playing with them pointless and unproductive but unenjoyable (I quit after a few seconds with them).  Why are there so many douches out there that think every game they play is death match?