Just a MINI-game.
First off, this game is a lot of fun! Between the decent variety of weapons and melee attacks you'll be hooked for a good while. However, that is pretty much the game. Kill zombies for a high score. That's about it. As mentioned before, this is just a mini-game.
The music and environment's are not bad but it's difficult to pull of that signature atmosphere the series is known for on a portable. Graphics are actually good for the 3DS though.
Just as Operation Raccoon City was average, perhaps even below average, Mercenaries is about the same. I appreciate Capcom for trying to tie us over until Resident Evil 6 gets here but just make sure it's worth the $40 you're paying for just a mini-game.