Good game, simple concept
I love the look and feel of the game. The sounds are nice and the graphics good.
Its a shame then than it gets boring so quickly, there is very little variation in the levels and the upgrades only count for so much.
Maybe you could slowly evolve the game through the levels..so i starts with black balls and then they change to charging spearmen..to jeeps to well spaceships or something. With another few buttons to guide a rotating shield or something.
That being said, that would take away from the simplicity and elegance of the game and would probably make it much more like another game i've played with a flaming ball on a string that i can't recall right now.