It's absolutley amazing. Anything I post will sound like fanboy gushing but DAMN, it's just a fast-paced beautiful ride. I wasn't even doing the missions just blasting cars over a few blocks and trying stunts. The are some jaggies on edges (mostly on the building you aren't looking right at) but I can multi-clusterbomb a whole line of cars and there is no slowdown for me. The controls and movement are very clean, I didn't have a sinlge "I wasn't trying to do that" moment. The climbing and ledge grabing is just ace, you can bullseye anything you want to land on. Jump from the 30 story building into the top of a lightpost, sure, easy.
I like how the health isn't regeneration in this game. You can get health back fairly easily by pulling in power with L2 (even from cars' tail lights!) and you are back in the game. The difficulty is tought to judge from the demo, you get full versions of the powers you have (not all powers though) so dropping a huge truck is pretty simple. It won't be like the in the main game I know.
most said phrase while playing the demo - "oh yeah, I am sooo climbing that thing"