Wow! What a cool game. Its what they call an "MOOG" (Massively Open Online Game), very similar to an mmog, except there's not quite as many people as in FFXI. Yet, there are a whole lot of people on the island simultaneously. There is so much to do and see, its unbelievable. You have the entire island of Oahu (and then some) to drive and explore with the ability to challenge anyone you happen across. The driving action is the best I've seen since Forza, and for all intents and purposes is a much better game in my opinion. There are maybe a few less cars available, yet there are plenty of cool rides to choose from. At present, I own about 60 cars and motorcycles with more available which I intend to acquire.
The only draw backs so far are the silly noobs who try to challenge people who happen to be in inferior rides. (e.g.- some jerk challenged me in his Enzo Ferrari when I was driving a "D" class Mustang...) Yeah, that's not happening:) Also, some idiots have figured out a modem glitch similar to that in Halo 2, which allows them ungodly times in the multiplayer challenges. (I refuse to sink to their level.) You can choose not to do that crap and still have quite a good time.
Other than that, its one of the truly fun games available for the 360. Its replay value alone makes it well worth the $40 + tax that it cost me. I would recommend this game to all my friends and anyone else who asks.