Having just beaten Portal around ten minutes ago, I can safely say it's a great game. My only real complaint, and it's not much of one, is that the first 17 or so puzzles are too easy, at least in comparison to the latter ones.
Oh, and I hated Room 18:
The end puzzle is not for those easily succumbed to the whims of motion sickness. And I did it on the PC; I sympathize for those using a controller.