Summary
The best online (and off-line) racing simulator. Period.
The Good
This game is by far the most realistic driving simulator around. The graphics are great, but the car physics really take this above the status of just being a game. If only you could feel the g-forces, you would swear you were controlling a real car. The amount of customization of car components (i.e. suspension, gearbox, etc settings) will satisfy any car nut out there, and for the ordinary weekend warrior, the preset settings work well enough.
The Bad
The cars are mostly imaginary, due to licensing restrictions (the game is made by an independent developer). The car sounds are computer generated (as opposed to being made from real recordings), and thus sound too flat and droning. The AI (for off-line racing) is not great, but the fantastic online service more than makes up for it.
The Bottom Line
A very fun, and accurate to real life racing simulator. The best there is.