Also, the Sword keeps on generating to the right after you press right, but if you press left it stops generating after releasing the left arrow, so you have to keep pressing left.
An occasional health pickup would be nice.
Maybe some sort of countdown for the firing rate? It's ok without, but it makes it harder to assess when you will be firing.
When picking an upgrade all movement on the background should come to a full stop quickly. I like the delay effect, but I have the feeling stuff keeps moving, preventing you to react properly.
The boss feels a bit unfair, it's really hard to avoid the bullets and with the focus circle shot it's virtually impossible. Maybe after a few movement upgrades ...