I'm actually in the process of converting our Segment.io docs to use it right now :) makes it way nicer for everyone* folks to just be able to edit Markdown, but still have the power to do lots of custom things to make the experience better.

* I was going to say for "less-technical" folks but then I realized that even technical people shouldn't have to be subjected to our current tangle of Jade files!

I agree, nice for writing/generating specs! Plus, check the raw documentation in markdown/markup/whatever you want into source control, right alongside the code, and reap all those benefits too.