replayability is insane, you can finish the bad ending without really using the gimmick of the star but once you start mastering it the whole game become a puzzle game where you try to figure out what are the most optimized star ride on each room, and that the whole level design was made around it.
The physics of this game are probably the best of any nes platform game, just play it this is a technical wonder

Based on the reviews I read, I thought this was going to be a miserable experience, but it's definitely the best NES game I've played so far, with the best platforming and everything felt responsive. Many people said the bosses felt unfair, but all you have to do is learn a pattern, similar to the fromsoft games of today, and the music and visuals were pretty amazing for this time period.

Very fucking hard. I think the final boss took me an hour, and that was just getting the bad ending. But, it's short and gorgeous enough to get away with it. This level of difficulty is a lot more palatable with only six* short levels and such wonderful graphics and sound throughout. It really is a technical marvel.