The first and foremost thing to remember when cosidering purchasing Forza Motorsport is that it is not Gran Turismo 4. Odd comparison, you may say, but many people believe it is the Xbox equivelent.
Despite both claiming to be racing simulators, they play very different styles. Forza caters for all levels of ability; with the addition of the racing line to alert you of the best position and speed on track at that given time.
All the other usual assists (ABS etc) are there, as are a multitude of cars (albeit only a quarter of that offered by GT4). The uprgrades and customidsations are endless but are costly and it wont be until later in the game when you have enough to really "splash the cash".
The Online mode really hold its own with the ability to customise many aspects of the race. To sum up, if you like driving sims, this is the only Xbox game that fulfils its promise, if you like arcade style racers, this will be a bore.