Excellent, 3rd person, wreak-havoc-style, sandbox game with a terribly short story.
While there is a campaign, it appears that your main objective is to destroy enemy installations like broadcast towers, SAM sites, radar dishes, etc. To unlock main missions, you must accumulate a certain amount of chaos points by destroying these enemy installations or doing faction missions. (there are 3 factions that you can work for, but it doesn't seem to matter which one you choose to do the most work for) Accumulating chaos points also unlocks weapons/vehicles that you can purchase and have delivered right to you via the black market. (you can also upgrade these weapons with collectibles you find throughout the island)
As far as gameplay is concerned, you'll probably find most of the fun involves spreading destruction with hijacked helicopters. Outside of a vehicle, you can shoot enemies and also grab opponents with your grappling hook and either throw them or tether them to something. Yes, this means you can tether an enemy to a vehicle and then drive off dragging them behind you until they expire.
Aside from the wanton destruction that you create, there is a storyline with cutscenes. Unfortunately, the cutscenes where you talk to people are kind of cheesy and don't add much to the game. It's a fairly cliché plotline with stereotypcial "Go America!" characters dropped into a foreign, island country. Also, the story is only 7 missions long!!!
If you do the minimum amount of side missions plus the 7 missions, you can finish the game in under 10 hours with 15% or less total game completion. Other than it being fun to destroy stuff, there really is no additional incentive to continue the game after that. (maybe Steam/Xbox360/PS3 achievements if you're into those)
At least the graphics are pretty sweet and the sound is nice. With a nice graphics card, I never had any framerate issues and I had a pretty massive view distance.
Ignoring the shallowness of the story, the game is still super fun. Also, it's price has dropped quite a bit since the the initial release, making it easy to recommend this game.