There are two issues: stability and which kernel interface.  Stability
seems pretty good; the problems are in venus really, not the OS
support.

I run coda on NetBSD.  The 1.6 branch still has the pre-realms kernel
interface (but a patch is available), and 2.0 (tagged but not
announced) has the modern interface.

At one point there was a performance advantage on BSD for the server
with respect to how anonymous mapping for RVM worked.  But this may
well no longer be true and my memory is fuzzy.

The coda kernel support has changed fairly little, and you can
probably port the netbsd coda support to openbsd if it isn't already
there.  Beware that some of the vnode locking rules have changed in
netbsd since the openbsd split (VOP_LOOKUP, I think), and getting
those right is really the hard part.

-- 
        Greg Troxel <gdt_at_ir.bbn.com>