Any game you can jump out of a helicopter in mid air, and hijack another one on the fall down gets 2 thumbs up for me.
Literally every ten minutes you can't help but perform some amazing stunt, like jumping off a skyscraper right as an artillery strike hits it, or driving a car laced with c4 right into a giant oil container. The possibilities are almost limitless as is the the amount of fun you'll have playing this game.
Mercenaries 2 takes sandbox games to a whole different level. The Coop gameplay is nearly flawless and 100% seamless. It takes all of 5 seconds to jump right into your friends game and start blasting everything in sight together with no lag whatsoever.
9.5/10 from me. (Make parachute DLC please!)