The level system in Scoregasm game is pretty neat. The game can be difficult, but since you restart at the same stage you died at, you essentially get unlimited continues. Also, balancing use of your gun vs the close in scattter shot type weapon gives the game some strategy. The close in weapon recharges as you kill things, so if you spam it too quickly you end up with no weapon for a short while, usually when you really need it.
Also, if you like these games, give Bullet Candy Perfect a try as well. It's by the same developer and is a lot like the old arcade game Robotron.
It's available from the developer's web site PWYW DRM free for $1 or more: http://www.charliesgames.com/wordpress/?page_id=14
Or on Steam for $4 if you prefer: http://store.steampowered.com/app/6600