Good
But too difficult, I hardly ever reached the boss in level 3. Maybe if you made it so he had temporary invincibility after being hit once by a bullet or explosive instead of multiple hits that can get you thrown off of the level and into the abyss below.
I liked the upgrades, but the bomb gun was too tough to use against the floating objects. Maybe if the bombs flew further from the captain instead of just in front of him in straight shots. Other than that, the weapons were good.
I think there were other things that needed improvement, but i cant think of them at this time. I'm sure someone else will mention it in my place though. Overall, this game was good.