Well done overall, cute graphics, clear gameplay. I particularly liked that the lesser guns were still usefull even after you bought the laser ('cause the laser can't hit the helicopters).
slight issues: the upgrades still light up in the upgrade screen, even after you've bought them all.
no ending other than a screen text, disappointing.
just before the end of each level (when the crawling zombie goes by) armor and fuel levels disappear, though you don't die, for no obvious reason.
Well done overall.