Quite a bit of fun
I should mention that there is a minor bug. If you manage to cause your ship to die *after* the level ends, then you'll start in on the next level when you respawn, but will not get a chance to put in your upgrades. Having that happen at the end of level 20 was kind of annoying, but I still managed to win.
I discovered that dodging along the sides is an absolute necessity on the final boss, but the boss is relatively easy once you figure this out. Once all his cohorts are dead, it's just a matter of paying attention. I managed to get through the game without insanely powerful shields, instead choosing to focus on firepower, with a touch of engines. This was no conducive to getting high scores though, as I found myself having to restart levels many times. I'd probably have put the game down in frustration if you hadn't allowed unlimited replays of the same level. I really appreciated that particular feature. The more hardcore people going for score can still restart every time they die :)
Overall, I'd say this was an excellent diversion. Amazing job.