I'm 51/60 puzzle pieces in - er, I'm probably wrong about that. Better to say that I know of 9 puzzle pieces I don't have, and I'm confident that when I get those last 9, I unlock the next section of the game.
It's definitely making me think in ways I haven't before, which is what Raph Koster says is the point of games. Mission accomplished.
Some of the spatially-proximate games are similar enough that solving one gives you a step up on solving the next one. I'm having a rough time on these last few puzzles, but so far, I haven't found a puzzle that seems unfair.
Need another review? http://www.computerandvideogames.com/article.php?id=194733
Go, download it now. I can wait.