I beat it last night. So awesome! I'll probably try the co-op tomorrow sometime.

I was impressed with the hidden (if you didn't look at them) Achievements throughout the game. (SPOILER)I got the one where you take the cube out of the room by accident and decided to follow the unreasonable orders given to me later in the game just to see if there were Achievements attached. I had a good laugh when I jumped to my death after Wheatley told me I should.