Right, well, yummymelon pretty much said it all...
I love this game! The atmosphere was really well put-together; the music and the graphics matched perfectly, and I loved the little voice.
As an "arcade" game, this is great. Its repetitive gameplay actually works in its favour, the rate at which the difficulty increases is steady but challenging, and the variety of enemies is numerically simple but complex enough to provide an engaging experience.
However, this is more than just an arcade game, because of the Upgrade feature. That was a very nice touch! It provides continuity without the demand for plot, like a role-playing game, and allows players to feel as if their points are achieving more than a spot on some kind of e-peen glory board.
Speaking of achievements, my only suggestion there would be to incorporate a -tiny- bit more descriptiveness in the Achievements section. A little humour wouldn't hurt, or maybe a little info on the varieties of enemies to provide meaningless but fun science fiction text. Hell, it's not even a necessary improvement to gameplay, that's how minor the suggestion is.
I have zero complaints about this game. Well done!