If you are a hardcore driving simulator fan, than this is a buy. If you are not, then try Gran Tourismo instead.
Good:
Great physics, awsome graphics, great sound, near perfect A.I., very challenging, arcade mode is better than carreer mode, great customization, XBL may be its best feature, and its the utimate simulator on the Xbox.
Bad:
Physics are taken too seriously, difficultly is taken to seriously for a racing game, carreer races are more of a chore than fun, one mistake on the track can mostly cost you the whole race, you are racing more against you'r driving skills than racing against you'r opponents, you will have to commit to tuning you'r car if you want to have a chance in carreer mode, the controller wasn't mean't for this type of game, if set on a higher dificulty, than get ready to spend A LOT of time tuning you'r cars, and this game is more frustrating, unforgiving, and more work than fun.
Result:
Well. This game is very impressive for a driving simulator, but ruims the fun by taking itself way too seriously. If you don't like strict rules throughout most of a racing game, then you will NOT like this game career mode (I don't). If you don't want a game that takes simulator too close to real life, than stay away from this game.
If you want a more arcadish style simulator, than you should give Gran Tourismo a try first because Forza Motorsport is NOT for beginners.