Such a cool little game! I really enjoyed this actually - I'm a sucker for upgrade based games anyway so this felt great. Some points that would be nice - some of the upgrades didn't give a huge impact and didn't feel like much changed. It's always satisfying when upgrades make a big difference, either visually or in terms of how it feels so I missed that satisfying feeling of feeling faster/more in control/better. The art was simple, yet effective though and I loved how to background got slowly darker the higher that you got. The high score line too was an awesome touch.
Overall: a great game, really enjoyed!