GNU Groff is terrible and the only reason it hasn't been replaced with a superior implementation is that all the better versions are politically incompatible with GNU. It's no surprise they're having a hard time finding someone interested in effectively code-laundering and slapping a GPL on it.
There are a couple of other troff implementations in use today.
Mandoc is very new and focuses on providing a complete solution for a system that uses manpages: it renders manpages in the terminal or to HTML on the fly, provides a database for semantic search of manuals, and is probably the second most common manpage renderer in common use (after Groff). It’s the default renderer on OpenBSD, FreeBSD, DragonFly BSD, NetBSD, Illumos, and Minix. Unlike the others, it’s not a complete troff implementation; it focuses on manpages (“mdoc”, “man”, “eqn”, and “tbl” macro sets).
Heirloom troff is descended from Sun’s troff, and focuses on nice typesetting supporting various PDF and OpenType features.
Plan 9 troff is, well, Plan 9’s troff, and is used by plan9port to render its own manpages.
There are a few other troff implementations (Neatroff, etc.), but these are the most widely distributed ones these days. I personally use Mandoc for everything except PDF output, for which I use Groff.
I've looked at the kind of PDFs heirloom troff can make, on an iPad, and my eyes, just, wow. There aren't words. If Debian would put heirloom troff in its base-install (along with, say, E.B Garamond or Junicode) instead of groff, there'd probably be more people (1) making beautiful man pages, and (2) viewing man pages graphically (with evince or another PDF viewer) than ever before. It's not like there's a licensing problem, AFAICT.
I have not seen a PDF made by mandoc that made me react that way. :-|
Yes, PDF output is one place where mandoc is not as good as groff/heirloom yet. (The other major one being support for generic preprocessors and other macro sets.) Mandoc works very well for terminal output, HTML output, and semantic searching though.
