I'm not ready to review Portal yet, as I've only completed the story and not all the bonus challenges. However, the game is quite simply the most spectacular puzzle/platformer game to date.
I do warn you, do not play the game if you have just returned home late in the evening after initiating a sugar rush via refreshments provided after an hour and a half long presentation by you and the choir prior to which you practiced for an hour. You will get nauseous under these conditions. In simpler terms, Choir Practice + Vivaldi (and extras!) + COOKIES + Tired + Portal = Nausea.
The following is over a year old, but effectively captures what Portal is about like any good preview should.