Eh, didn't really like it that much. Puzzles were disappointing, mainly because they were composed mostly of inert scenery with very few portal-able walls. P1 forced you to think about puzzles and there was usually more than one solution. By contrast, P2's puzzles largely take place in environments that are almost bereft of portal-able walls and you spend the majority of the time scanning the place to find the one wall where you are actually able to place a portal. I often got stuck in P1 even though I had all the pieces, I just couldn't put them together right. In P2 that never happened, when I did get stuck it was because I simply overlooked the one bit of smooth wall that I was supposed to put a portal on next. And it was very easy to overlook walls in P2's much more varied and cluttered environments. About half the time you're not really thinking about what you're doing, you're just portaling to the only places you can portal to and the solution presents itself. I was also disappointed by the characterization of GLaDOS. In P1 she was wonderfully quirky and loopy, you really got a sense that she didn't have all her marbles. In P2 she's just a jerk.
On the plus side, P1 was woefully short and this is pretty much more of the same, even if it is just a sequel. Overall I'd say about as much goodness as P1, but spread out over a longer gamplay time with some bits that feel a bit like needless padding (especially the middle bit, where you're just going through test chambers with literally nothing happening other than some childish taunting at the start and end of each test). You get a reunion with an old friend and another fantastic song at the end.
which often require quick reflexes
Point to one level in Portal 2 that required quick reflexes. Just one.
The end part when *spoiler* tries to crush you with spiked plates several times, for instance.