The basic mechanics were fun, but the lack of ammo respawns meant that anyone aiming for a high score would simply run in circles for most of the game without shooting, which doesn't seem to be the point of this genre. That said, the "high score" system didn't seem to be working anyway.
Ammo would also sometimes generate on the side walls, making it unreachable.
I didn't immediately realise that only the central circle of enemies could be hit, and not the "wings". That could have been explained better, but then again I figured it out pretty quickly.