I really liked how the music changed depending on which character (power) you were using, and how they all meshed with the background track really well. The same with the hats.
The game is pretty simple and easy, though it kind of feels like Reimu, Marisa, and Youmu are the Easy, Normal, and Hard modes respectively. Reimu is pretty strong.
There were some visual bugs, but the only real game-ending bug I encountered was when I was playing as Youmu and picked up a Marisa orb. The sprite turned into Marisa and then I nothing came out when I attacked.