The difficulty curve is spot on, and with some polish and and some graphics work this could become pretty popular. The controls do feel solid once you get the hang of it, and it is a pretty unique game. Only change I would make would be to just have the cube on the mouse, because I died a few times restarting the level and then having to move my mouse back to grab the cube, then moving it to where I wanted it. Other than that, I think controlling something that controls the player is a very good idea, and could possibly make a good mobile app, well done. :)