Pretty fun, but not quite perfect.
The game was great in pretty much every respect, but there are a few ways in which it could be improved. For instance, the "whee!" sound the aliens make when killed isn't particularly satisfying and tends to become a little annoying after a while. Some additional instruction as to how to play the game would also be useful. The controls also felt a little sluggish, and the game felt a little slow at times. Finally, why isn't there some music? Maybe you could check the audio portal to see if anyone has a remix of the original theme. It's an old Russian folk song, so it should be in the public domain.
Everything outside of that was well done. I do like the idea and the game is fun to play.