This game is incredible. It is a great game, especially if your having a party with other people. The best thing about it is that it uses Xbox Live to its fullest. You can race against other people, you can send/gift cars to your friends, etc. There is an auction house where you can sell your cars on live to other people. It uses an ebay sort of system.
Customizing cars is great. You can do upgrades on such things as paint jobs, handling/braking, engines, etc. It is really realistic and that is another key feature of this game.
In the career mode, you win money by winning races, which you can use for upgrading your car, buying new cars, and using the points on xbox live in the auction house.
Overall, a very fun game and I recommend it to anyone.