Double Jumper I found it challenging
and I liked it, except for the distance between checkpoints, which as you said, makes it harder, but it also might encourage someone to quit earlier because they're forced to redo a level that had already taken them forever. Another thing I wanted to point out was how suddenly the ball can change direction, which I found to be incredibly irritating. Maybe you can add a bit of gravity so the ball's a bit easier to control and a suggestion for another type of block would be one that moves so you have to time your jump correctly to get onto it.