It's really really great, and I can't wait to play co-op with it when I finally get a new controller.
I've completed it but I don't want to spoil anything for anyone, so I'll use spoiler tags.

(SPOILER)I kinda went off GLaDOS a bit, I think she talked too much, and had too much of a personality, but that's ok because Cave made up for it. Cave Johnson was the best part about this game (and that section of the game was the most interesting part), Wheatley was also pretty hilarious. All in all, I was very pleased with this game, and I can't wait to run through with Developer Commentary on.