Somewhat inconsistent, but great overall.
True, some puzzles are really easy, even to the extent of giving you one or two extra pieces for no reason, like level 23 for example. (Obviously the early puzzles are supposed to be easy, so I'm not counting those.)
But many of the puzzles are challenging, exceptionally interesting, or both. Levels 16 and 20 are a couple examples of really cool puzzles, 16 in particular having at least 3 reliable solutions as far as I can see (though one is much faster than the other two and thus the best).
Add to that an overall solid user interface (the number of games that lack FX-mute and music-mute buttons is astonishing) and this is an easy 10/10.