Needless to say, the comparrison to GTA is everywhere when talking about this game since GTA is the new spokesmodel for open ended games.
If you enjoyed GTA for its open ended, free roaming, nonlinear enviroment, then you'll love this game. If you loved GTA for running down pedestrians, then you might not want this game.
In a nut shell, you're a mercenary in the near future. You are sent in by ExOps, a secret organization of mercs for hire. You are in Korea to stop a powerful enemy whose tyrany must end.
Before the game starts, you must choose who you will be. You can be the brutish marine who is good for beginners as he can take more damage than most. You can be the one who loves things that goes boom and go fast. Or, for experienced players, the stealthy and sly female merc.
In Korea, it's always a war, which adds to the realism and action of the game. If you played GTA3, you'll be familiar with the idea of factions. You can do odd jobs for different nationalities and persuations, but be careful, sometimes a mission can go against another faction and ruin your rep with them. However, you can get it back by doing missions for the hurt nation or bribing them. :D
The map isn't that big, but its easy to get lost on the roads. Missions are fairly easy and calling in air strikes and vehicle drops and supply crates is always a blast. However, as you progress (especially during the Ace missions), it can get pretty hot in there and can be difficult to manage through. So make sure you have cash to call in supplies to heal up.
Overall a terrific game. Nothing I can find to change about it without ruining the idea behind it. It's at least worth a rental.