Playing through the first one was in my top 3 most satisfying game experiences. Great game, then suddenly there was a huge twist. Then that ending.
The second one is great but has such a different feel. And I wish they'd gone with "GladOS has actually been activated and bored this whole time" to explain why there was so much extra STUFF. And I didn't want to hate Wheatley. Etc.
Still, all the Cave Johnson stuff was so great. And great puzzles. And the whole potato thing. And finding out GladOS' backstory...