I was put off but the demo, but am glad I tried the full version!
The story follows classic hero Tanner and his partner. Chavez (from Driver 2?) was scheduled to be transported to another prison. Things dont go so well... He has a breakout plan and succeeds. So the main task of the game is to re-capture Chavez once and for all.
In Driver SF you can purchase and upgrade your own vehicle (having the new Ford Challenger is a sweet treat).
Most important is the "Switch" mode. This enables you to get an out of body experience, and switch between vehicles, which is a major help in chase pursues.
When I played the demo, it seemed the graphics were mediocre, the driving physics were terrible, and the switch ability just seemed ridiculous. Since then I picked it up at the red box to give it another spin, in full final build and I am glad I did. Now I bought it used at Gamestop :D