We finished Portal 2 tonight. Overall, I'd rate it as good as the first one, although it didn't evoke quite the same level of surprise and delight. The gameplay was fun, the puzzles were challenging -- and enough different from the first game, while having the same flavor -- and the humor was in full force. Doing the multiplayer cooperative with T was great, too; there aren't many games we can play together like that, and it makes a nice change from having one person watch while the other drives. I wish we had started with single-player mode and moved into cooperative afterwards, though -- it would have flowed better, I think.
Also, GLaDoS is still the best videogame villain of all time. (Of all time!)
Definitely recommended if you liked the first one, and/or if you enjoy a good puzzle game with a lot of sly humor. The only downside is that it still seemed short to me, although this is probably because I'm so used to RPGs that last billions of hours. Then again, I'm not sure how much longer this could have been without feeling repetitive. So, thumbs up, way up, and I'm already starting to wonder if they can pull off a third.