Nice one! For some reason I liked this game quite a bit, may be the mechanic, may be the charm, may be the difficulty, I don't know. It do hit a sweet spot on the arcade game difficulty and length. As for some critique, I'd say the red balloon, which is the first enemy ever encounter, is way to quick and strong, so that you easily lose on the very first level, and that is not ideal. Also, using 1,2,3 to select upgrades is a bit odd. We are using the mouse for most of the time anyway so why not use the mouse instead?
Anyway, good job!