So here is my spoiler-free review:
The writing was great. Never as fresh as the first game, so rarely as funny, but still packed with great lines and moments and with a good overarching story.
The mechanics were good. There were just a few flaws that dragged it down from being a glorious masterpiece.
1) Bizarre, inexplicable signposting issues, usually in transitions between chambers. Too many moments that require you to spin in circles trying to figure out where to put a portal (for example, through the grate on the floor in one chamber). These wouldn't be an issue except for the fact that the signposting is normally so GOOD, so you just end up befuddled and totally thrown out of the flow. It's as if Mirror's Edge has abruptly stopped highlighting things red for you. This is frustrating because the first game also had these issues in the sequences when you'd escaped the test chambers.
2) Portal conducting surfaces are used too stingily. A comic I read (I think on Nerf Now) really helped bring this into focus for me. The first game tended to be very generous with portal conducting walls. This game generally just has a few of them in very specific places. This makes the puzzles more about looking for the obvious white conducting surfaces and trying to figure out what to do with them than about experimenting with your portal placement and trying to get it right. In the end this makes puzzles less satisfying because the space of possible options you can explore is reduced - it's not so much that you're building a clever portal system that works as you're discovering the solution intended for you. That's less fun.
3) Generally I just felt like it'd been slowed down a little from the first game; they talk a lot in the developer's commentary about how they were worried it would be Too Hard. That was probably a good move in terms of making it more accessible, but they lost a lot of the really kinetic, wild moments, like hurling yourself through a momentum transfer and then firing a new entry portal to get a better angle on your second exit trajectory, or using turret rockets to take out rocket turrets. Also gels were lame.
Those are basically the issues I saw. The ending was great though.