Difficulty curve was a little wonky, but co-op in general had some cool ideas. Not all of them were stellar, but they definitely made us think in general. The puzzles are the strongest part of Portal 2, so I don't mind blasting ~40 of them without much break for story or anything, though it's noticeably a little dryer. Teasing a boss fight and then just ending on nothing (for DLC) was kind of lame.