I love all things Python, but for some reason I could never wrap my head around ReStructuredText's syntax. Maybe it's because all my favorite sites use Markdown (including this one!), but I find Markdown's syntax to be more intuitive.
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I don't really understand why reST is so unpopular compared to Markdown.
There are two reasons: first is that Markdown is much simpler and is more narrow scoped and second is that reST implementation only exists in Python and Haskell - to some extent, not supporting extensions, obviously. Both of those reasons make entry level higher.
It depends on what you're doing. For documentation I'm always running reST but for articles for sites etc (with a static site generator) I will turn to Markdown.
However, generating PDFs, a static website, and any other formats from a single reST document is much easier than doing to equivalent from an HTML document so it has its uses, but I would not use it as blogging markup unless I planned on publishing a book from the same blog content.
There's also a bunch of things in Sphinx that need improvement with the PDF and ePub output.
But for what it is and does, I like it a lot. I don't know of anything for Markdown that lets me do as much structured markup as I can do in reST.
Markdown is so much simpler than Rest.