Undoubtadly the best game to ever grace Sega's 16-bit Genesis, Gunstar Heroes was, and still is to this day, one of the best simple minded action games ever produced.  Bearing resemblance to Konami's Contra games and other "run 'n gun" type games, the gameplay is pretty basic.  You run around with automatic firing weapons blowing away the hordes and hordes of enemies that get into your way.  You'll find plenty of different power ups that quickly change the pace of the game, but the two player mode is where the real fun of Gunstar Heroes is to be had.  For back in it's day, the graphics were great, and today they still hold up for a 2-D side scroller shooter.  Controls are incredibly easy and you know what to do from the minute you pop in the cartridge and play.  The sound and music is the game's only lowpoint, but most of that can be blamed on the Genesis' poor soundchip (remember, this system was known for having shabby sound).  All in all though, Gunstar Heroes is still one of the greatest action games of all time, and if you still have a Genesis and don't have this game, you should do all you can to track it down.