This game is deceptively great. The mechanics are incredibly simple and the puzzles get fairly difficult later on, but there was only one level that left me feeling frustrated (one of the tight and narrow moving block levels that close off two of the tunnels. It could just be me, but it seemed like the pattern was random and it was a matter of luck if you got a good shifting configuration when you started the level). Otherwise, every time I failed a level it felt like it was never a fault in the game, but more a fault in my timing or skill level.
Even if I failed a level 20+ times, which definitely happened (or more. I'm looking at you, Final Level!), beating a level always felt so satisfying. With the exception of the shifting level I stated before, that was only kinda satisfying. Mainly because I didn't have to deal with it anymore.
Fantastic game! I can't really offer any constructive criticism because I don't know how you could improve it, other than making it all shiny and fancy and adding sound effects/music/etc. Thanks for a very tense game!