What could be better? Once you're stuck, it's impossible to see the outline of the individual pieces. So in effect, you have to move them all off the board.
There are 600 levels - way too many. Also, in order to do the Normal levels, you have to first complete most of the 150 Easy levels.
There is something very relaxing about this puzzle. I did complete all the easy levels and moved on to the normal, hard and challenging levels. At the moment I've solved 60 of the 150 challenging levels.