I just beat this. Bought it a few days ago to play co-op with my brother, got most of the way through singleplayer the first night (and the first 5 hours of the morning), and finished tonight. I was really bugged by how much of some points were just "Find the one panel that you can put a portal on in this giant room", and I feel like the gel test chamber was designed poorly, since the only thing I could figure out was turn everything white and then see where it gets me. But it was a lot of fun for the most part, and starting right around the part where he kills you, it gets downright awesome.
I have to say I was really surprised at the ending, when GlaDOS actually saved you. I didn't see that coming.
The boss fight was a lot tougher than the first one, or maybe I just was having a hard time performing under the time limit when it is more complicated than rocket + portal = victory.
edit: Also, this pretty much describes quite a few points in the game for me:
http://awkwardzombie.com/index.php?page=0&comic=050111