Great game, love it.
level 21 is fucking impossible
the game is nice, but there should be a way to disable the "flashy effects". Resetting a level is distracting and takes too much time and computing resources (my computer stutters when rendering them). At the second difficult level where I had to reset many times this became so frustrating I couldn't continue playing anymore.
Dragging stuff around is nice on touch screens but not so comfortable with a standard mouse, when you have to do it too often. I would have gone for a hybrid mouse+keyboard interface, in which you select the tile to move by hovering mouse on it, and then move it with arrows.
Only a minor interface flaw that I noticed: the "Congratulations / You solved all puzzles" screen is shown for the first time after completing level 41, even if the nine puzzles after it haven't been touched.
It's great for passing the day by