I just couldn't let go of this until I finished it, and found all the bonus stars. Because of that I unlocked the secret ending on my first try, and I was pleasantly surprised that the game sets you on the screen just before you trigger an ending, so I was able to check out the regular ending too, without replaying the entire game just for that.
I also liked the hint system a lot. It just shows what the end of the first step should look like, but not how to get there, or how to actually solve a puzzle.