The flow between difficulty is off. But its fun!
The gameplay is sound, but for the secret boss, it is just impossible.
The only way I can actually get a shot in is by having half of my life be obliterated!
Whereas the last 15 'hard' levels were a piece of cake.
Here are some things I'd reccomend if you plan on making a sequel.
Add a story, it adds more of a soul to the game and it makes everything not seem so spontaneous.
Add more levels! The xp bonuses are pointless, if you can quickly rack up 3 out of the possible 3 levels by the first boss!
Make the bosses flow from least challenging to more challenging. The first one was a breeze, all you had to do was focus the cursor on the red area and dodge away from a distance. The next boss however, was just cruel. Not only did you have to fight him on lower health, he was spouting bullets like they were candy!
Add a more rewarding gameplay style, like gaining money for destroying asteroids, and using the money for later shop areas or rest stops to buy upgrades or better armor, or emergency health packets, etc.
This one's always the way with me. No matter how well the graphics are, improve, improve, improve. I'm not saying you should just focus on graphics than gameplay(*cough* Too Human *cough*), but eye candy is always a plus with me!