I like the concept of this game, and it's fairly fun, but the controls are simply too twitchy. The main challenge of the game quickly becomes keeping your weapon from spiraling out of control.
I like the idea behind the controls, though. It looks like you're using a gravitation or a spring function between the weapon and the mouse. You might consider adding a simple damping term; it can go a long way in making things more controllable. Even a simple 'velocity = velocity * 0.9' should be enough to fix the problem. It would make the weapon settle down when left alone for a second or two, making those 'spazz spirals' a lot easier to recover from.
Bottom line: keep at it! This is a pretty neat idea for a game mechanic. It definitely makes a simple defender game more interesting; it just needs a little tweaking to be more playable.