I played the demo just now. A few, quick, things i would point out:
The simulation mode while driving is really fun, the cars behave and look quite realistic and that's why it's a bummer that you can't shift the gears manually. Since you cannot control the force applied to the throttle, or as with the manual gear changing, at least the rev count, you end up doing wheelspins most of the time. Although the way the car behaves on uneven surfaces and different road conditions makes it really fun to drive around in high speeds.
The physics are nice, since you can knock of, shoot through and destroy most of the things around you. There were a few, not so good, moments. When you wreck your car completely, you can get out and fix the damn car in a few secons
? Then there are moments where you hit a wall, flying through air at 80 miles an hour, and then walk off just like nothing happened (though you will die if you drive very fast into a wall, or another car).
The driving A.I. is attrocious..... Police cars driving strait at you no matter the situation. They will follow you everywhere, even if you jump off a ramp (which i did).
The shooting and fist fighting is a mix between Mafia and GTA 4.
I would also point out, that in some situations the game seems to start lagging really terribly for no apparent reason (it's not my hardware, i'm positive).
I didn't spend much time with it, so i can't comment on some of the things, but it feels like the game isn't finished yet.