I have finished the game and collected all the crystals even though I have not collected all the hearts and thunderbolts yet nor I have done all achievements (I will continue playing), I felt like I needed to write a review.
First of all I am really amazed how well made this game is. It has a special element that even if it is truly difficult,challenging and requires skill, you somehow get addicted but never become bored or enraged when you fail. The more times you fail the more motivated you become to keep trying. You can play it for hours, have fun and enjoy every single moment of it and end up losing the sense of time (hours become minutes). For me this is the characteristic of a really good, well-made game.
There is only one thing I didn't like which is the "Checkpoints" option in level-select. I tried to complete the "No Safety Net" achievement thinking that you should just go above the checkpoint portals and ignore them(which was really challenging in some levels), so thats what I did until level 16 when I realized that there was an option for checkpoints! So what I was doing was completely pointless! It drove me mad xD ! What made it more difficult is the fact that I was collecting the crystals at the same time too! So maybe it would be better make it more obvious or make a message with this "checkpoints" option appearing before starting a level.
I read @fuckoffasshole's comment saying "-don't try to have both crystals and no checkpoint beaten level. Ever." So thats ofc challenging but not too difficult to achieve except maybe the last 3 levels and especially level 17 which is hell.
Also I am not sure if this is a bug or not but on Level 7 if you press repeatedly and quickly the UP button when you spawn, you will be able to avoid the red area and fly up to the next area. This is a big shortcut for this level.
Anyways that's all I have to say,now I go do the rest of the achievements. Thanks for this amazing platformer and I would be really happy to see more content from you.
Good job and keep it up! -5/5