Good platformer, very entertaining to watch as a speedgame. That said, gating chapter 9's plot progression behind the so-called "optional" B-sides and blue hearts feels like a huge mistake. You can do all the other story levels with just the "normal" A-side difficulty (chapter 8, conversely, has no plot whatsoever) so having to turn on assist mode and cheat through optional levels just to get the last bit of the story doesn't feel good at all.