I think this game would really benefit from a boss; the pattern was extremely linear with the sky units and foot soldiers coming the same, so a boss would benefit it tremendously, especially if the boss' were all different.
The upgrades made the game way too easy as you ended up with quadruple bullets with max output of damage as well as four rockets that took out ground units, meaning you only had to aim at the corner and you cruise through it.
The speed ability was more trouble than help, as enemies end up all over the screen at a very rapid rate. The art was well done and as Almond had mentioned, new bosses would be pretty fun.
The overall rating i'd give it is 4/5 stars.