Not a bad attempt at an open world war simulator, but it has it's share of problems.
Technically, the game is obviously too much for the Playstation 2. I haven't played any of the other versions so I don't know how well they work but I have played the Playstation 2 version and I can tell you it's riddled with technical issues. There's frame-rate drops, short draw distance, pop-in and some of the longest load times I've ever seen. I've heard the load times aren't as bad on the other versions though. It's not a bad looking game at all, but it isn't exactly Shadow of the Colossus either. There's little to no art direction, it just looks like a generic shooter for the most part.
The soundtrack is actually pretty good. The beating of the drums can really get you pumped up and ready for combat. the voice acting is surprisngly good as well. The soundwork as a whole is a pleasant surprise.
The game-play is where this game both shines and fails. First of all, I have a problem with the title. Playground of Destruction. There isn't a whole lot of destruction except for exploding vehicles and it's not much of playground. It's relatively big, but there isn't anything to do if yu're not doing a mission. This leads me to my next complaint. The missions are not only highly repetitive, but the instructions are also terrible. There were several times where I had no idea what I was doing. The dificulty is really unbalanced. It starts out easy and then shoots up greatly in difficulty after about an hour. The combat feels ineffective. You shoot someone, and you see them go down, but you just don't get that solid feeling you do in other shooters. What is really fun though, and sets this game apart from the others is when you get to call in air strikes and blow the crap out of your enemies. It's too bad the environments aren't destructible. The vehicles work pretty well, they have a very arcadey feel. In terms of game-play Mercenaries is pretty mediocre with the exception of blowing crap up.
A pretty slid game that will satisfy your need to blow things up and terrorize your enemies. Not all that much of a standout but it manages to stay afloat with solid soundwork, a good if not uninteresting open world, and allowing you to call in air strikes. It's worth a look if it sounds like your thing.