Great fun, so well-polished and simple to understand, hard to master. I think the difficulty of the main path is good if you're okay with running into a few tough ones, but the extra puzzles are where the real nasty ones can be found (I love the real nasty ones).

Some of the tougher levels had the issue of either being too complex to reasonably understand with the tools provided (slow-mo helps, but not enough in some cases) or easy to bypass the complexity by putting all the rails down where they need to go and fiddling with a couple directions to get it working, without having really understood what you were trying to achieve.

The music and art is lovely and somehow the little postcard at the end of each world was a nice enough incentive to want to see it through.