A lot of people have already said this, but I'll say it again: THIS IS THE BEST RACER ON THE XBOX, possibly anywhere else. I've played them all...Rallisport, Sega GT, even GT3 on PS2 (which so far had been my favorite). This game is so polished, feature-rich, and varied, that you will never want to put your controller down. When I heard about the Kudos system, getting points for sliding and things like that, I was very skeptical. "Haha what a gimmick, trying to draw people in with some stupid lil kiddies' trick!" Boy was I wrong. The Kudos system adds an incredible amount of longevity to this game. On a scale of 1-10 (with Elder Scrolls being an 11 haha), this game has a replay value of 10.5. No joke. You will actually want to master all of the courses to improve your worldwide ranking. You will want a higher Kudos ranking and more tokens. And doing all those tricks and going through cones etc...IT's JUST DARN FUN! I've never enjoyed a racer as much as this, and in my book, Gran Turismo can step aside.
Note: If you have this game, you MUST get Xbox Live! It's still a great 9.5/10 game without it, but with it, it's a 10. You'll see what I mean, this title makes perfect use of Xbox Live and will definitely set the standard for all other sports games and racers from here on.