At this point, everyone who plays games knows that Portal 2 is one of the best games of the year and most people who are interested in it have already played it, but I do feel it's worthwhile to say a few non-spoiler things about it.
First and foremost, if you are at all interested in the game, go and play it right now. Don't put it off any longer. As with the original, the surprises are some of the best parts of Portal 2 and the longer you go without playing it, the greater the chance that you'll have them ruined for you accidentally or intentionally.
While the potential for replay is about the same as it was in the first game, you'll definitely want to play Portal 2 on your system of choice as you only get to experience it for the first time once. For me, that was the Windows version, which looks downright awesome with all the video settings maxed.
Play through the single player experience before playing the co-op. The reasons for this will become obvious once you do.
When a bit more time has passed, I plan to write a more detailed piece on this game, but it's impossible to do that without spoiling at least some aspects of the experience. Bottom line: If you think you might want to play this game, go play it. It's great. If you haven't played the original Portal yet, it's not required to appreciate and enjoy Portal 2, but you should play it as well given the opportunity. It's also great. :)