A great arcade racing game. Not as good feeling as the Gran Turismo Series.
The largest complaint I have cuts right to the core of the game. What is this game supposed to be? If it's an arcade racer with the ability to mod, then it excelled. If it is a sim type game that will allow the player to feel like they are really driving these vehicles, then they unfortunately failed. To me, the cars all feel sort of the same. Some are faster and some are slower, but honestly the differences are quite small. In gt5 you really felt the differences between the vehicles when driving. You feel the pavement below you. You feel connected. You feel like you are driving these cars. and it is awesome. In this game, it is more FUN, but less awesome (if that makes sense).
Forza 3' AI is GREAT! People are total jerks, and don't just stick to their lines, like in GT5. Forza 3 allows you to tweak a lot of the elements that add challenge to racing.
In the end (to me) Forza 3 is a very fun in depth Arcade Racer, but should not be compared to GT5. Not that one is better than the other, just that they are aiming at 2 different gamers.