I don’t have much to say that can’t be said better than Oli Welsh’s conclusion to his Portal 2 review over on Eurogamer.
Portal is perfect. Portal 2 is not. It's something better than that. It's human: hot-blooded, silly, poignant, irreverent, base, ingenious and loving. It's never less than a pure video game, but it's often more, and it will no doubt stand as one of the best entertainments in any medium at the end of this year. It's a masterpiece.
Though, I do feel end of this year would be better worded as end of time.