If only my school playground was this much fun.
There isn't much about Mercs that can't be liked.
If your ever going to make a game that players will be playing for a while, about the best way of doing this would be making a sandbox game, filling it up with people, missions, vehicles, buildings to destroy and so on and let the player go nuts. And this is what Mercs does. There is so much to do, find and destroy, you won't become bored for a long time.
Another of mercs strong points is the humour sprinkled throughout the games various aspects. In the dialogue is where it is especially good, containing enough wit to make it fun, but not too much to keep it serious, and therefore keep you on track.
The mission structure is also one of the most unique I have come across. It's based on the 'Deck of 52' that you chosen merc (you have a choice of 3) is tracking. You have two ways of finding them. The first is wandering around looking for them, or you can do missions for one of the various factions who will then give you intel to find one of the deck of 52. Each person on the deck will give you then give you enough intel to go after the aces. Certainly is enjoyably different, something that was sadly omitted from the (somewhat lacklustre) sequel.
Re-playability also stands this in good stead. As mentioned before, you have a total of three mercenaries to choose from. But instead to just giving you three different skill sets, each has there own dialogue based on there own personality. They also speak a second language that's also unique to them, and it just so happens that some of the factions don't always speak English, so depending on which faction you visit, mission your on and merc your playing as, you will be able to glean a little extra bit of dialogue, some of it useful, some of it darn right amusing! Ahh, the humour!
Also for the eagle eyed, there are references aplenty, mainly to star wars, all done in the humorous manner that's threaded throughout the game.
The game also features what called "The Merchant of Menace" (this pun I like!), which, for all intents and purposes, is a online shop full of military gear. You can order new weapons/ammo (machine guns, RPG's, etc) and vehicles (helicopters, tanks, buggies, APC's and even a super car!), all instantly delivered by helicopter. And then there's wide variety of air strikes to aid you on you destructive travels. And There not all the same thing just with a different name, they all have a different application. It helps so much on your rampaging antics.
These are all so refined, buffed and polished, you can't help but fell a respect for the games developers Pandemic, who are sadly now defunct.
Only one thing comes to mind, that being the difficulty curve which rises fairly sharply at about the halfway mark.
The Bottom Line
Honestly, I think it maybe good enough to call "the rough guide on how to do a fun military sandbox game", it is that enjoyable. At the very least, a PS2/Xbox must buy.