Played through Portal 2 from start to finish with friends late last night. It was much better than I was expecting. I thought the first game was had interesting mechanics and gameplay, plus a nice bit of a charm, but no real storyline to speak of. This is obviously completely different in that regard -- which is good, because by the end of our 7 hour playthrough my brain was really hurting from all of the puzzle solving so story sequences were a nice break.
There's a ton of room for future content additions too. I still want to see a chamber that uses Thermal Discouragement beams/redirection cubes, Light Staircases, Tractor Beams, all 3 gels, and companion cubes in 1 puzzle.
I only got about 2-3 hours into the co-op before I had to sleep, but it was pretty satisfying too.