Finished Kero Blaster some days ago, on the standard, Zangyou (hard) and Omake mode. Starts pretty simple but it gets harder, with a few tricky platforming sequences as well. Hard mode is totally worth playing as story (what little there is) is different, levels are almost completely redesigned, bosses have a few new patterns and attacks, and the last level and bosses are completely new. Omake mode, on the other hand, is just the standard normal mode with a boss rush sequence at the end. So for anyone who decides to play it, don't stop there once you beat it, play Zangyou mode.
Overall it's a pretty fun and (eventually) fairly challenging sidescroller, certainly different from Cave Story, but worth playing if you like the genre.
