It does need some work, there are a few bugs, but the concept is pretty good. I love the upgrades idea, you should add more. Perhaps even weapons in time, but they are not needed.
As the previous reviewer said, its repetitive, and it is. You need to find different ways to keep the player interested, bonus rounds, other ways to kill them, bosses at the end of every level, etc..
I also felt the game was very slow. 5mph, everything felt so so slow... I though a 15mph was a 'nice' speed, and 30+ was extreme with boost. That would of been better I think!
Now about bosses, there great, I enjoy the fights, but there way to long, and there a bit difficult, the first lady was knocking me good, Usually she did no damage to me, but then suddenly she'd hit me several times and I'd be almost dead, then come after be for the blow, in general I was able to dodge it but sometimes it was to difficult, ending in my death, and it was only the first boss too. Though it was a bit too hard though, for that many hits, I could see it being the 'final' ending boss! When I'm trying to kill someone for 15-20 minutes it gets tiresome. I also realize they regenerate shields, kind of good, but would be better if they had a ship more like yours, fast regen, low hitpoints maybe?
Also, the bug I found is you can sit on the bottom left of the screen, and most of the time the ships would end up blowing up without you doing anything, sometimes one or two would survive because of a civilian ship moving them out of the way, but in general you could simply sit there until the end.
But the concept as I said is awesome, you need to expand on this, and I hope you do, I'll definatly play! It is a pretty enjoyable game either or, just needs some tweaks!