Everyone says this game is great, but I have a slightly impartial view on Mercenaries. I greatly enjoyed the first half of the game, but the second half didn't hold my attention nearly as well. I don't know what it was...but the game just got frustrating after a while. When you take 10 minutes to get to the mission area, blow up a bunch of stuff, then simply get run over by a jeep 4 times in a row - you lose a lot of interest in a game. I did finish the game, but the cheap deaths really got to me on the second half of the game.
Aside from the cheap deaths I would say the game is good. The controls are pretty good, the graphics are nice, and the explosions are AMAZING. They are easily the coolest looking explosions I've ever seen in a game, and they really add to the experience.
I was also disappointed by the length of the game. Reviews I read said it would take 35 to 40 hours to complete it if you did everything and I did it in around 23 hours, and I did do everything.
The game itself is very fun, but there were a lot of disappointments to be found. I'd suggest a rental, but it's not really worth a purchase.