Very cool game! The movement system was slick and the world was atmospheric. The dynamic audio really did wonders. Could probably do with being more difficult, the only times I died were because I stopped paying attention rather than a need to improve, even before upgrades.
The flight seemed pretty irrelevant compared to the other upgrades, it just started so much slower and the flight orbs weren't common enough to be worth choosing them. I also think the + bonus to particular upgrade options was pretty unbalanced, it was always choose whichever had the most plus signs (other the the aforementioned flight), so the only choices were when there were no bonuses or equal bonuses between several options.
Overall though I really enjoyed it, you did an excellent job!