I played the original Portal several months back (I think it was on sale, or free for a short time). What was seemingly a simple puzzle game, gradually introduced more story and humor. Makes you think, while keeping you entertained. I loved it. It's a short game and can leave you craving more.
So I was excited to pick up Portal 2. And last week I just completed the single player campaign. Took me 9-10 hours, and I was actually sad to see it end. With new game mechanics, more complex environments, and an even deeper back-story to uncover, I think it's even better than the original.
The acting (for the robot voice-overs) is well done and performed by actors you might recognize like Stephen Merchant and J.K. Simmons. You're continually using your portal gun to solve test chambers to get to the exit, but the story and voices make sure you never get fatigued.
Plus, the game saves your progress after each puzzle, so it's a good game to pick up and play whenever you have a little extra time to kill. But if you're like me, you'll probably be up all night, always saying "one more test chamber, and then I'll call it a night... ok, maybe ONE more..."