Had a bit of trouble with the controls (I generally prefer platformers to split the movement hand from the actions hand), but gameplay felt pretty good.
The double or nothing mechanic was interesting, though it took me a bit to figure it out, mostly because there's no indication that you've now gotten the doubled upgrade -- having some kind of message would probably be good for that. I ended up mostly avoiding the speed and jump height upgrades, since they seemed to be a bit detrimental, in that it makes it a bit harder to navigate between all the obstacles as things progress.
The shield seemed good, but I couldn't figure out where it showed how many shields I had left, so it was hard to keep track. I.e. a lot of my gripes are feedback/information that's missing. Otherwise, quite fun, and cute art!