It's a great game, a lot of cars (not as many as Gran Turismo 4, but still alot), very good customization options, and good graphics (although DiRT's graphics look better). Online play is decent, but very few matches actually race (many are drifting, dragging, and destruction derbies).
I think it's better than GT because you don't have to waste time getting licenses, and the auction house is really cool, if you have enough money. Also, if you have friends with Forza 2, they can gift you cars, which is good if you don't wanna buy them 😀