Best car game I've played. Live only makes it better.
The ability to play this game on any difficulty in the single player mode allows you to open all of the tracks an pick up many different cars and to use those cars in many different conditions. Playing Live you can help your single player because you also earn tokens, like the single game, that allow you to purchase cars to use in either Live or single player.
The graphics are the best for a Live game that I've seen. Playing during the day you'll see things from dust kicking up from your tires to the reflections of your car in windows of other cars or offices you drive by. At night the pools of light from the lamps will reflect off of your car as you drive through them, and you'll learn not to have your own headlights go out because of a head on crash.
The sound is very good from the engine nosies to the multiple sound tracks avalible, or your own if you have one. If you have 5.1 dolby you'll hear cars coming up from behind you and either the left or right of you.
The game is a keeper. Rent it if you have doubts, and if you can try it on Live with some friends, you'll have a blast.