I gave the custom levels a try, and they work fine, except for the level preview, which doesn't load. I also tested the level editor, and it's perfectly usable, but has some issues:
- Editing splines can be a pain in the ass, especially when adding in new points, cause often it'll make the spline illegal, since their tangents are always horizontally aligned. I'd suggest allowing illegal splines while editing, but not when previewing/saving the level.
- Boosters and spring could "stick" to the surfaces of platforms without the need to perfectly position/rotate them.
- Support for Del. and Ctrl+Z would also be nice.
I also played the main game, and it seems like it's improved a lot(I haven't played it in a long time, so I don't remember what it used to be like): the controls are more conventional, and new features like rails are very fun to use.