Woah, this game is incredible! Definitely the kind of game I'd play over and over again until I get sidetracked by something else. The art style actually gives me a throwback to a flash game I found a few years ago where you had to shoot down asteroids that were hitting the earth.
My only minor complaint is that there's a lot of waiting inbetween bullet waves - on Normal at least, I haven't tried Hard - but other than that, this game is absolutely stellar, and I'd love to see an endless mode!