I finished it just about an hour ago, and... it was amazing, hands down my favorite game. And this is just single player, I've yet to touch co-op.
The humor was perfect. I loved GLaDOS in the beginning, she actually reminded me of Cassandra- the so-called 'bitchy trampoline'- of Doctor Who. I loved the automated voice, and the fact that Aperture was so prepared for the apocalypse, including some kind of animal or turret king. I loved the bit with the deer. I loved the 'if the laws of physics don't exist, god help ya' line. The Rat Man drawings and the one den of his I found- the art was lovely (and the moon phases were an interesting touch).
I loved POTaTOS, the whole bird thing, the... near confirmation of one of the leading theories about GLaDOS and Aperture, everything. Even the ending- I had it spoiled, but I honestly could not figure out how it could happen. But it did. And it was awesome.
The puzzles were great (and I managed to solve every one on my own, I couldn't do that in the first Portal, whether this is smarting up on my part or dumbing down on Valve's is up in the air), the ending song is... I like it more than Still Alive, it's far more... gloating, biting, whatever you want to call it. GLaDOS has well and truly won here.
The design was amazing too, I'm glad I have it to myself until Monday (until we get a new copy, it's a shared game among the family), I need to do a run where I just slow down and look at everything.
Again, the only real complaint is the length of it- it just leaves you wanting more, but it's great to see it go out on a (rather literal) high note than burn out its welcome. I can't wait to see where it goes next.
ETA: It's also nice to see Chell catching up to our friend Gordon in terms of doing things no sane person should ever do.
Lunacy, man... Lunacy. (... of course, that bit also gives me the urge to go play Hedgehog Launch. Again.)