My initial opinion was just shock. Mostly because they expanded the portal world far beyond the testing chambers, and managed to send you down to the bowels of the facility, through old testing rooms, and back to the ending. (attempting to not spoil too much stuff). The story was in my opinion far more immersive than the first, and was a great play. I'm going through co-op with my brother, and it's great to see that theres yet another smaller storyline to follow there.
Also what impressed me was the fact that the game wasn't repetitive really. I was worried I would get bored with portal puzzle after portal puzzle, but they really made you think in this one. Unlike the first one, I actually got stuck a couple times, with little to no idea how to advance.
Portal 2. A gem of a game.