I actually just bought it and so far my only complaints are the ones that Bukowski already mentioned. The voice acting is awful. Other than that I love just running around parachuting and grappling excessively. It takes forever to make your way from once side of the map to the other, and it seems like there are an endless amount of things to do. The game is a good mix between GTA and Crackdown.
It's single-player only. The story is generic and the voice acting is terrible.
Those things aside, it's the best open-world sandbox game I've played in quite a long time. Massive world, tons of stuff to destroy, unmatched sense of scale, and there's a ton of game there. The grappling hook and parachute combo are fantastic fun.
In other words, despite the game's shortcomings, I'm having a ton of fun with it.