I've spent a bit of time playing this. Overall, the levels are as good as the designer makes them. Nothing has quite come close to traditional Nintendo designed levels, but some are quite entertaining. The most amusing thing to me are the mystery mushroom cameo levels.
I played the 100 Mario challenge for a bit yesterday and it was a mixed bag. It gives you a selection of random levels and you have 100 lives to get through them. Some levels were fun, while others were frustrating troll levels that had doors opening into to pits. I'm hoping that the level quality will get better the longer that this is out.
I also tried making a few normal courses. The interface is quite easy to use, but levels turned out to be quite bland and boring. More than anything, it made me appreciate the care that is required to design a decent platformer level.