yogsloth: I can report that "Marble Mountain" is an absolute joy.
My boys (age 6 and 7) have just plowed through it and are up to the "Castle" level. Unfortunately, there might be a game-breaking bug on that level, as there is one place where the marble gets stuck. The game isn't finished as several levels are noted to be "under construction", but what we've seen thus far has been balanced and fun. Hopefully I'm wrong and there's some way around that stuck point. (It's on a platform just above the hedge maze where you have to roll down to the left and "jump" over a couple of gaps in the pathway.)
Enthusiastic thumbs up from me.
Nice! Thanks for the impressions, I think I'll bump this up to the top of my "play with kids" queue.