Pretty fun! Very smooth controls, nice visual effects when firing the gun, satisfying sound effects (although, as you note, somewhat too loud). The action gets pretty furious pretty quickly.
The environments look snappy, but a bit more variety would have been helpful. Sometimes the enemies appeared to shoot through walls--I'm not sure exactly what was going on there. Also, enemy pathfinding would get stuck pretty easily. I used the slow-mo ability a lot, but it's hard to say how much it really helped me.