Well the whole of my
www.afterfive.co.uk website's "developed" in XML -- and it's all downloadable including the XSLT scripts (
http://www.afterfive.co.uk/guide/latest/xml.tar.bz2) if you wanna' take a look.
The great benefit is that the XSLT stuff makes transforming the content from XML to XHTML, RTF, LaTeX, Cerocwiki, etc. formats a doddle -- as long as you *think* about the problem first -- and have a good DTD (yeah, I know I should move to schema, but I'll wait for a bit first).
I use Saxon mostly and occasionally xsltproc.
In the past I've been able to take structure documents to/from databases really easily -- which means you get the benefits of both file and database based systems, e.g. it's really horrible trying to spellcheck data in a database, and it's not that easy to get a SQL like query from XML -- but having a decent structure made it easy.
I've been starting to look at XInclude which looks like it could make life easier -- plus, also looking at SVG, and that's definitely looking a fun way to make graphics easily in XML.
Cheers,
SpinDr.