It's bigger than GTA and probably deserves just as much recognition.
What matters most in this game is the control, the power, the weaponry. You play as this character but he is in a nutshell, a model for you. YOU play this game. YOU make the decisions. Lara Croft and Sonic The Hedgehog go around doing what they have to do to get what they want whereas in this sandbox game, like many others, it's up to you to decide what you want and how you're going to get from your safehouse where you are currently situated to where you want to be. This could be from driving a super-fast heavy drop motorbike to parachuting around the island like a crazy lunatic.
The environment isn't perfect to look at some of the time, but it's beautiful enough for the Xbox 360, the story mode is a bit of a chore but after that, the islands are in your hands. Get started with those achievements!