I quite like the idea of the game - encouraging fast gameplay by rewarding kills with invincibility - but I feel as if it falls short in some regards. First and foremost, the invincibility granted by killing someone is far too short, and feels almost insignificant. The speed of enemy projectiles means that, even if it was halfway to hitting the player already, invincibility has already worn off by the time that it reaches the player. This means it's difficult (although I may just kind of suck at the game) to get more than one or two upgrades before dying. While difficulty is certainly a factor in making a game enjoyable, too much difficulty makes a game frustrating to most players.
Also a minor audio bug - whenever I change my computer's volume level it restarts the soundtrack.