Really easy, it needs levels :D
Once I figured out the tricks it all became easy though. Don't know how to improve on that :(. Still, it was good.
It's simple, entertaining, looks nice, controls well. The only complaint I can level against it is that the music is way too loud. I appreciate the mute button, but it could use some balancing. The keyboard shortcuts are nice. I would consider adding some difficulty levels that expand the field between 3x3 to 6x6 or so. Maybe also add a leader board based on quickest times. Those would cool additions.
I never really liked these kinds of puzzle games. Their only trial and error and offer nothing else. But if you like that stuff than its okay.