We've been having fun with this since the weekend:
If you decide to try it, you MUST read the "Information" section first (and refer to it while trying to solve the puzzles), and you really should sign up for the Forums. I wouldn't have been able to solve most of them without the forums.
So far we've made it to Level 25. I'm ashamed to think of how many hours I've wasted playing this game, but once you start figuring out the puzzles, you don't want to stop.