My playtime with Gunstar Heroes has mostly been with the GamersGate (DRM-free) version. So my Steam stats don't reflect my actual experience with Gunstar Heroes.
+Gunstar Heroes plays like Contra on meth!
+Non-linear stage selection, a'la Mega Man.
+Modest hardware requirements.
+Gamepad support (360, Logitech, and generic).
+Two player co-op.
+The PC version of Sega's old line-up features save slots and filters for smoother graphics.
+Save slots from other versions of SGMDS/SGCS (Gamersgate, DotEmu, or Amazon Game Downloads) carries over to Steam's version. I tried it and it works.
-4th gen (Sega Genesis/Mega Drive era) graphics -only if you're a graphics snob.
-No online multiplayer...yet.