As I said on another board, I managed to beat both the single player and co-op in two sittings.
While I liked what a played, while I really enjoyed the story, writing and humor, I thought that the game was too short and too easy. The fact that you are very limited to what you can and cannot shoot portals on greatly cuts back on experimentation and challenge--not to mention fun. I think the co-op portion of the game is better from a design stand-point at the very least. Sadly the co-op is over too soon and many of the elements I had a lot fun with, like the gels, are barely used up until the endgame.
Don't get me wrong, this is a good game, but not great. It's a victim of hype and pricing. I'm glad I was able to snag the game for $45 from Best Buy (with a $10 giftcard to boot) because otherwise I'd be feeling ripped off. Here's hoping that the user created levels on the PC/PS3 can breathe some longevity into this one...