That's everything you need to know really... this game combines the massive environment, factions, and mission based paradigm of the GTA series with modern military technology, a semi-plausible political scenario in a post-coup North Korea and you have an instant classic.
The licensed Havok physics engine and the experience of the Pandemic team (with some help from the Battlefront & Full Spectrum Warrior teams) make Mercs a blast to play. Everyone will have a different path to success and every mission can be won in any of many, many ways. Blow up 4 NK barracks? You can use an RPG-7 and extra ammo. You can plant multiple C-4 charges. You can drive a Chinese APC with a missile launcher on top and shoot them. You can capture nearby NK howitzers and depress the guns enough to hit the barracks. Or you can call for artillery strikes. I'm sure there are 40 other ways to accomplish the mission, all just as much fun.
The gameworld is huge, and filled with plenty of vehicle to hijack. You can sneak through enemy territory in disguise until an enemy officer spots you, or you can barrell through at full speed running down anyone who foolishly stands in your way. The map is filled with challenges you can play over and over to sharpen your skills, earn bucks, and improve your scores.
Every vehicle in the game can be driven or flown. The Helicopters, APC's tanks, jeeps, civilian cars and trucks etc... The game is laugh out loud fun and has nearly endless gameplay options.
I just can't reccommend this any higher. There have been some great games this last couple of monthes, with Halo 2, Mechwarrior 2, Prince of Persia 2, NFSU 2 but this is an real winner.