One of the best games ever made!
Even though you can do all the missions and follow the stoy directly, most people will like to spend time causing destruction (but watch out, you lose money for civialian deaths). You also have a helpful PDA which allows you to view a map, shop for weapons, supplies, vehicles, ect. view your stats, and see if you are either friendly or hostile with all of the factions. In addition to tracking down Song and the Deck of 52, you can find hidden weapons of mass destruction blue prints, ancient treasures, or destroy listening posts all of which can unlock items in the shop, money, ect.
I think the graphics in this game are pretty good. The explosions are very detailed and fun to watch. The physics of objects are good to, though there are scripted AI deaths and the falling buildings are scripted as well.
Many people say they find lot's of annoying glitches but I have only seen one (my guy got into a rock somehow and could not get out) The game runs at a pretty good frame rate also.
Overall Mercanaries is a great game that is a must have for all free roaming action game fans.