A fine game. Having played through dozens of puzzles and reaching the final difficulty, I can attest to its high replay value. I'm also impressed with the visual presentation.
The one thing I'm not a big fan of is the scoring. It takes me considerably less time to score a given number of points while playing lower difficulties than it does while playing higher ones. But I nonetheless appreciated the delay between unlocking difficulties, as it prevented me from getting in over my head while I was getting acquainted with solving techniques.
I would tweak the scoring, but everything else is very nicely done.
P.S. Including advanced-level hints for the final difficulty would be a help!