This is by far the most technically accurate driving simulator I've played. I bought it primarily for the chance to drive the Circuit de la Sarthe and drive the Audi R10 TDI and although I've not yet gotten the R10 yet, the Circuit de la Sarthe is pure virtual bliss! The single player experience is one of the best I've played of any racing game and the seasons and tracks and available cars make for hours of fun.
The variety of vehicles is varied enough to keep anyone engaged and the ability to nearly infinitely customize not only the mechanics of the cars but the actual set ups as well in terms of suspension tuning, spoilers, tires, etc. In addition, the in-depth ability to customize the looks of the cars is awesome!
But, it's not all fun and games on the Circuit de la Sarthe... I am EXTREMELY annoyed with the lack of ability to create one car design/paint scheme and apply it to multiple cars. This is disastrous! Why spend all the time to create a great design for a car and only be able to apply it to that specific car???? Ideally, you could create a great paint/graphics scheme and apply to any car... it might need to be tweaked for that car but at least the basics would be there. The second disappointment is the multiplayer experience... you cannot play this with a friend on your own xbox... the only multiplayer is via xbox live which, in my opinion, is great for Halo, GRAW, etc., but mediocre at best for racing games...
If not for the disappointing multiplayer and the car design limitations I would absolutely have rated this a 5.