Well, I sorta beat it...
Given the difficulty of this game (and therefore the time already spent in it) I'm not too motivated to search for the secret levels. It was kind of fun at first, but soon became VERY meticulous and unforgiving. Checkpoints were a nice touch; However: disabling a 10 point achievement for having 1 checkpoint halfway through a level that takes about 100 tries to complete? Really?
Art and music both get my highest marks. And even though I bitch about the difficulty, it's at least done on purpose. No "fake difficulty" due to poor programming here! Your controls are fluid and consistent. Hitboxes were a little big at times (especially the flame jets), but once you get used to it, it wasn't a big deal.
If you made the secret stages unlock for, say, beating the corresponding boss without checkpoints, that might motivate me to continue playing.