Amazing, Genre Defining
Some of the new features in this game are:
1) the ability to ride bicycles, combines, and many other new vehicles.
2) swimmming! No more flailing and drowning!
3) Driving between the 3 cities, through hillbilly country
4) Can physically alter your character with tattoos, haircuts.
5) You are what you eat: you can gain and lose weight!
6) Flying airplanes, from remote control to jumbo jets
I really am enjoying this game, having played well over 50 hours and completed less than 70% of it. The main storyline takes well over 30 hours to complete, and that is only if you don't do any of the side missions (taxis, ambulances, pimping, firetrucks, vigilante, trucking, etc.)
The storyline is very involving, and your character has more depth than earlier games. (Heck, in GTAIII your character couldn't even talk!)