Absolutely amazing! It really exercised my game designing chops, and by the end I almost wanted a notebook to take down tweaks and changes I would make were I designing the game. However, to serve its purpose as an educational tool, I wouldn't change one iota.
One thing I thought was really interesting is that my initial assumption was that the fastest, snappiest controls would feel the best, but I ended up going with controls that were fairly constrained to find that balance between feeling good as well as challenging.