I noticed there was a little bit of demand for my opinion on this last night in chat, so here's the thing.
So what is this? The title describes it pretty well. It is a modified Mario level that, once you start it, completes itself. Here's some good examples:
http://www.youtube.com/watch?v=Huy6UewpyUI
http://www.youtube.com/watch?v=WAgOIKdW810
This one has a surprise ending!
http://www.youtube.com/watch?v=PKMSWevlIdQ
And of course, the predictable timing of such levels means people have used them to create a percussion track to various J-pop tunes. Here's Luigi playing Dan-Jo-Dan-Dan-Jo-Dan-Jo
http://www.youtube.com/watch?v=T4bNRVVixvo
And of course to the inevitable question, what's my favorite?
It is actually a five-part series, the first of which can be found here:
http://www.youtube.com/watch?v=owJuxBhjf7U
The designer of these levels clearly put a lot of effort in making them clever.
If you have a NicoNicoDouga account, you can view the original here (in all its commenty glory):
http://www.nicovideo.jp/watch/sm656895
There's a lot more videos like this on NicoNicoDouga than on YouTube.
What do you think?