Mercs is one of top awesome action shooters for the system!
The game had two modes. There was Arcade, which is more straight the point with power ups and progression, and then original mode, where you can actually increase your max health and go through certain extra areas and levels. You could also purchase upgrades and buy more bombs if my memory serves me correctly. :P
Mercs was very addictive in the sense that there were huge military vehicles to blow up, fight at the end of levels, and sometimes even drive! The most memorable part was when you get the tank and you can drive around and blow the crap out of everything and everyone!
Throw COOP into the mix and you have yourself one of Sega's most awesome and bad ass shooters! :D