I quite enjoyed this. There's really no strategic reason to stop shooting, and there's no added challenge in having to hold the mouse button down. So I think it would only make sense to include an auto-fire option. But that's not so much a problem as it is a minor nuisance.
There is one big problem though. When you die you have the option to continue or quit. But you don't have any way to buy upgrades. This means that if you get stuck on a level you're SOL. It would make the game much better if you could go back and replay levels or buy upgrades between dying and retrying.
Other than those two issues this is a great game. I absolutely love the graphics.