It downloaded pretty quickly for me, for a big demo release anyways (about 45 minutes which is crazy long for 1.2gb, but not too bad). The graphics are unbelievable in every way. It does have some minor tearing (and I noticed very minor clipping in the first cutscene), but that doesn't detract from it at all. The tearing is about the same as in Jak 3, so nothing to worry about. You don't feel the framerate drop like in heavenly sword, just a little tear with a still smooth feeling framerate. The DoF effects are the best (coolest looking to me anyways) I have ever seen in a game and a definite improvement from the E3 gameplay videos and trailers (I only really play console games since the only PC game I play is the UT3 demo).
The gameplay is very very smooth and tight feeling. I was actually a little surprised at how fast and good it felt. Aiming is extremely easy, the cover system didn't give me any trouble (I never got accidentally stuck to anything), and walking over logs was A LOT easier than I thought it would be after seeing so many videos of people falling on them constantly. Maybe they made walking on logs easier than it was at E3, but like I said it's a piece of cake. It took me a couple tries (and an unfortunate death) to get the timing down for the hand to hand combat. But once I did it was very fun.
All in all, I was very pissed off when the demo ended. And that's a good thing. I always know when a game is good when I don't notice the time passing and just keep playing.
I couldn't have said it better.