Mercenaries for the PS2 is a LucasArts game that didn't receive a lot of fanfare, although it probably should have. It was a good enough game to garner an upcoming sequel on the PS3, however.
In Mercenaries you choose one of three mercenaries, each of which has a different benefit (speed, endurance, stealthiness) and try to capture as many of the "deck of 52" known terrorists in Korea. You work with, and sometimes against, the UN (renamed the AN here), Russians, South Koreans, and Chinese. Missions often force you to betray prior alliances, and you sometimes have to pay the price for doing so. Ultimately you are to stop a madman who has an arsenal of nuclear weapons. The game is broken into two large areas, four sets of loosely-connected missions, each culminating in an insertion into a smaller area to take out the main terrorist of that area.
The game is very open, sometimes even more so than Grand Theft Auto games. You are able to blow anything up with explosives, and call in air strikes pretty much anywhere. You can pilot a variety of helicopters and drive tons of cars, trucks, and tanks. You can even serve as the gunner in these vehicles if you so choose.
The storyline is a bit weak and as such the game focuses more on the gameplay -- the various tactics you can use to capture the terrorists. It's quite fun, and even after beating it has a lot of replay value. The game takes about 30 hours to beat from start to finish, so it's not incredibly long, but the openness of the game and the world makes it a very good 30 hours. Voice acting includes many veteran Hollywood actors, such as Carl Edwards and Bruce McGill, but the script is nothing amazing.
Load times can be a bit long, especially when first starting, that's about the only negative in this game. The game can be ridiculously difficult at times, but since there are so many ways to accomplish your goal, it's not frustrating as you can just try a different path or call in some support.
If you enjoy Grand Theft Auto style open environment, mission-based games and you haven't played this, pick it up -- especially to whet your appetite for the upcoming Mercenaries 2 on the PS3.