I just finished the game today. Some of the later levels have amazing design and creativity. I mean, really...a lot of it was extremely impressive, especially considering you can make it all with the creation part of the game. Speaking of that, while creating takes a long while to get used to (and creating takes a LONG time), it's fantastic, really. Even without servers being up, I'd still say this is worth the $60.
My only real gripe is one I've read other places as well: it's frustrating to have to replay the level if you run out of lives at a specific checkpoint, and the controls don't feel perfect. Jumping doesn't feel completely right...it feels like it's barely delayed and like something about the speed, distance, etc is slightly off.
That's about it, though.
And to think, MM could probably release more features/tools/levels via a patch down the road.