Personally, I enjoy it, quite a lot infact. I love the social aspects of it like the storefront, and the fact that everyone else who plays the game is as passionate about cars as you are.
Compared to Forza 3, the career part is much better too. In FM3, it was just race after race which quickly became tedious. In Forza 4 however, there are a lot more modes that break it up, like autocross, trackday modes and much more. Rivals mode online is epic too (Damn you Danowat!
In short, just buy the damn thing! It's fantastic fun, and I am sure you will love the physics.
Edit: Forgot to add, make sure you have plenty of space left on your hard drive. Forza 4 is a big game!