What a great puzzler! I had a lot of fun and I think for the levels I've played the difficulty curve is good. One thing I thought could make it more polished is to keep the music continuous between levels instead of reloading. I like the music too, BTW. Great work!
Edit: One thing I'd like is reset being mapped to R instead of having to use the UI (makes it heaps quicker when I know I've stuffed up).