Is troff/groff relevant anymore?

I am using troff for my everyday typesetting; I am using the Heirloom version of troff (see http://heirloom.sourceforge.net/doctools.html ) which has very powerful support for fonts (TTF, Type1, OTF, etc.), Knuth's algorithm for formatting the paragraph as well as several micro-typesetting features that you can't find in plain TeX; it is lighter than LaTeX and as long as you don't need to typeset equations, I find it much easier to get high-level typesetting than with LaTeX (it's much easier to load fonts, get control over the exact position of things, etc.).

I run/schedule 'canned' PDF reports using *roff, generating tables and simple PICs as I process the data with python, and organizing sources with simple bash scripts. Smooth & Simple.

Like vi, *roff is always there, ready-to-run and generates clean PDFs with minimal fuss. I like *roff's terse markup.

This said, I don't pretend to be a representative sample of typesetter/mark-up users...

I've heard that troff/groff have largely been replaced by TeX. Is this true?

The only thing that I know actually used troff nowadays is manpages. Is this also true?

I've only ever seen troff/groff being used for manpages; for everything else, people seem to use TeX or LaTeX. So I'd answer yes and yes.

If not, what are some other uses?

Apparently it was used to typeset books back in the day: http://www.troff.org/pubs.html
As that page was last updated in 2006, I don't think that it's still being used for this purpose :)