This game concept is one of my favorites, but it is being stretched thin. For example, Armor Mayhem. It's a great game, but the side scrolling shooter concept has been done to death, ever since the release of Unreal Flash. Raze 1 and 2, Strike Force Heroes, The Last Stand 1-3, etc have the same game-play mechanics that are quite similar. Again, all wonderful games, but "Been there, done that."
Solar Rift is also another good one on this site. Or if you want a very good one, Shadow Complex is something to look up if you have and Xbox 360. Very good game.
Horribly boring genre where every game is the same.
Let's bring out the universal weapons list!
-Pistol (Infinite ammo) -Shotgun -Machine Gun -Rocket Launcher -Sniper Rifle -Laser gun -Some other gun that may be somewhat unique.
Now, let's go over what you do.
Shoot stuff. Die. Respawn. Shoot some more stuff. Go crazy in small arena. Get spawn killed a few times. Change level. Repeat.
There's not any content to them. There's no real strategy, the enemies are all the same, the levels are boring and headshots are the only thing that matter.