I really liked the game very much. Awesome sound, graphics, controls, game design. Liked the step by step tutorial based thingy. I think (my personal opinion) that the gravity should be removed and the max velocity should be limited. Since when I am dropping, it's hard to launch the ball up. And when I am going up, swiping again boosts the ball too much.
I didn't get what the black ball was for. And I didn't even get what or how the rewind mechanic work. I would recommend that if you don't have time to add lots of features that's totally fine. But you should concentrate on the core mechanics and what is already implemented. Making the tutorial was an awesome thing and glad that you did it before hand. As you replied to one people that you didn't get time for level design. You didn't have to. The level design is already good. You might have spent a bit in play testing cuz the rewind mechanic is not working.
As the theme was "REWIND" and that specific thing was missing in the game. I am afraid that the game logically doesn't fit the theme, but again, everything else other than that is great. Great graphics, great audio, great guide and great controls.
Also I would really appreciate if you check out my game! :D