I've beeing using groff to nicely format man pages to plain text like so:
groff -mandoc -Tascii /usr/share/man/man1/ps.1 > man_ps.txt
but the problem is that you need to find the actual man page file, which adds another step to the process. This tip landed in my inbox the other day from the Unix Guru Universe Hot Tip of the Day list.
To convert a man page to text, simply do:
man ps | col -b > man_ps.txt
Much easier than messing around with groff and the mandoc options.
groff is still more useful for doing stuff like exporting to straight html like:
groff -mandoc -Thtml /usr/share/man/man1/ps.1