Very Hard (to stay awake)
In the game and in the author comments, it says that the difficulty scales up very quickly. I'll tell you that that is a load of baloney. There were only a handful of levels that required strategy and/or skill. Also, right after a harder level, there would be an extremely easy one waiting. That aside, I'm giving this a 3 because there were two bugs that I found to just kill any positive thoughts i had about this. If your ball lands close to a corner, instead of bouncing, it slides along the side as if nothing were beneath it. Other times, i would hit corners (including those of blocks in the middle of stacks) and bounce off in a random direction. On levels where precision was key, these two problems were only magnified.
In the end, it was a very boring game for me - but in all fairness I did play through and beat all 50 levels.
I think you should definitely consider reworking the levels to be as challenging as you say they are. By the way, it really couldn't have been improved graphically, so props for that.