It's an awesome game.
Things you can do:
1. Take part in organised single player races
2. Take part in organised online multiplayer races
3. Challenge other online players to impromptu races
4. Buy and tune cars & bikes
5. Explore the island, driving through cities and countryside with normal road traffic
6. perform other missions such as car delivery, package delivery, giving lifts to models and hitchhikers.
You need to buy houses to have garage space for all your cars and bikes, you have a budget and you earn cash by doing all of the above. You can also trade cars and bikes and many people only use this method to earn money to buy more cars for themselves.
Bottom line, if you have xbox live and like immersive game environments and you like mucking about in cars and bikes buy it. if you don't have xbox live i would question its worth. If you want a more structured game or a more realistic physics model, buy PGR3. Having said that, Atari are promising a "Hard Core" download that changes the physics model...