It's Far Cry 2 with everything most people (but not me) found frustrating about that game removed, and about a million collectibles, sidequests, and customization options sprinkled on top, if you like that sort of thing. Basic gameplay is at least as good as FC2 if not better (albeit also quite a bit easier even on the highest difficulty, because enemies go down quicker, there's loads more autoaim, you can buy red dot laser sights, there's no gun jamming, etc.).
I have some quibbles with it as a diehard fan of FC2: the UI is cluttered XP-grind dogshit after FC2's naturalistic take, and the whole thing is much, much more scripted and much, much more forgiving, so it loses a lot of the oppressive atmosphere of the previous game.
But it kind of fucking rules otherwise.
And it's huge. I've been playing for hours and I haven't left the first map square because I'm skinning komodo dragons and doing quad bike races and stabbing pirates for cash.