If you don't go in expecting a lot, you'll have a blast with this game.
The graphics are nice, but nothing to be especially noticed. Though there is very satisfying to blow down a building with an air strike and see a huge bellow of smoke, dust, and debris roll out from the falling mass.
The sound in this game is where it fails the most, I believe. It's very uneven. The sound effects are well done, but don't seem to be as loud or clear as some of the dialogue, and in the same fashion some of the dialogue isn't as clear as it could be. (Play with Mattias Nilsson and you'll see what I mean)
The gameplay itself is very simple and very fun. Mercenaries doesn't offer a lot to do outside of killing and blowing things up, but this enough to keep one entertained and satisfied. There are a few glitches here and there but nothing to really be concerned with. Very minor things that rarely happen.
If you can pick this game up used, do it. As I said, if you don't go into this game expecting a lot, you'll have a blast with it. It doesn't do a lot, but what it does do it does very well.