Excellent puzzle game and really hard. Sometimes frustratingly so (but that's good in a puzzle game)
For anyone trying for the "complete the game in 500 seconds" medal, I notice the game shaves some seconds off the first time you replay a level. So yes the "fastest" possible time the first time through is slower. Proof? Clear your cache and speed through the first two levels as quickly as possible. 5 seconds. Now do it again. 2 seconds. Maybe it'll help you in getting that medal.