I have had TDU since its release on the 5th of September. Through a month of racing with friends and others, it has convinced me to buy this game.
The best things about this game are its rather extensive number of vehicles. It has roughly 90 on disc and two for download. This may seem like a small number when compared to Forza or Gran Turismo but those 90 cars are the best and fastest out there. The one other thing that definitely convinced me are the game's great online features. Being able to cruise with anybody in the world is very amusing even if no one is a friend to you.
The two specific things that keep this game from being my all time favorite racer are: it freezes much to often when it's loading a shop or by you hitting the guide button during a loading screen and that the "massively" part in the game's motto M.O.O.R. (Massively Open Online Racing), should be taken out. Games in the future like Forza 2 for example will support more than 8 players on screen at once. Now these two things may hit you like they did me but they're not big problems unless you expect nothing less than perfection.
All in all TDU is one of the best games out right now for Xbox 360 in terms of online features. My warning is that if you do not have Xbox Live, do NOT buy this game unless you really love racing games. You may get bored quicker than online users and say this game got repetitive quickly. Test Drive Unlimited is like I said, is near perfect but rather "mindless" flaws keep it from retaining the highest score.
Lots of cars, lots of roads, and the ability to race anyone, anywhere. That is incredible. The graphics are great, but the physics at times are horrible. I prefer sim physics and this is arcade. For 40 bucks, you really can't go wrong with this title.