OpenBSD comes with an audited version of the
arla project's AFS client, built in. I would consider that client stable, by its very nature (it is not the latest, of course). Public AFS servers are even listed in the
Getting Releases page of the OpenBSD Project website.
Learn more about operating the client from
$ man -k afs.
The
OpenAFS project server's roots are an open source branch of IBM's AFS, based on the original CMU/Transarc AFS server, granted to the public when IBM took AFS off the market. Stable? AFS dates from 1983.
Start with the OpenAFS website. There is plenty of documentation there; the Administrator's guide will tell you what you need to know to architect a solution.