Played this through as a Christmas calendar (one level per day), in a similar manner Dualbyte mentioned earlier on. Was a nice way to wait for Christmas. I'll have to say though that the first two levels were insultingly easy (the first one only took 11 seconds for me, and the second 14 seconds), but they got more challenging after that, which certainly was welcome. The first two levels notwithstanding, I thought the difficulty level was about right - not a cakewalk, but not frustratingly difficult either. This was a nice puzzler, but I knew it forehand after buying and playing through the main version of the game.
Now just waiting eagerly for the DLC to arrive :-)
Happy new year!