This is a neat little game that is well polished.
I didn't notice any bugs, or any features that seemed unfinished.
The art fit the game style well; and I liked the upgrade options.
I also liked that I could shoot 2 or 3 souls at once if I placed the beam carefully.
:)
In terms of improvements, I agree with what other people are already saying: more variation, smaller gap between SFX volume and music volume, and gradually increasing difficulty.
Is there currently a win condition, or does it just go on forever?