First off, yes, it is as good as the critics have been stating (it's currently running as one of the highest rated games of all time on metacritic). It keeps the pace, stories and puzzles of Portal 1, while substantially increasing the length, and massively increasing increasing the graphics detail. Portal 1 though a great game was mostly set in simple grey boxes and had almost no animation; the sequel has the lush environments you'd expect in a current game, and in a nice touch many of the environments themselves are animated.
There are some negatives though: the single player game, while much longer than Portal 1, is still a little short and it doesn't have the equivalent of the challenge levels from Portal 1. There is a coop game (which is completely separate from the single player campaign), but it's a bit of a pain to play that (which is why I haven't yet).
The other negative I had is that there are some places where I think the puzzles are not quite as sharp as the first game. There is a little more re-use of puzzle elements, and there are a few too many that are basically spot the obscure place that you can shoot a portal. In fact, of the three puzzles I was stuck on for more than a few minutes only one was because it was an interestingly tricky puzzle; the other two were because I missed a portal surface.
One thing it does almost without fault though, and that is the voice acting (and the scripts for them). This is uniformly superb, and does a lot to bring alive a game that otherwise could feel a little dry.