just to follow up on the project and close out the thread:
we ended up using the ply-bead and are very happy with the results.
it's an old house, so we spent a lot of time preparing the rafters and joists to give us a pretty level and consistant surface (pretty level...in our old house, nothing is plumb or level or square, really!).
purchase a finish nailer for the job. it's worth it, believe me.
you were right about the but joints: they are very visible. we used 1/2" x 2" pine to trim over those joints.
our walls are drywall, so we needed to put up some moulding there (also to cover the gaps that we were left with). we used 1x4 pine for this. we bevelled and planed all the moulding, by the way. it was a quick step with a huge effect.
after all of this, I used some paintable silicone caulk on the lap joints and dabbed some over the nail holes along that joint as well. used the same caulk (tooling with my finger) to cover imperfections along the perimeter moulding.
once it's primed and you get a finish coat on there, it looks good. much more character than drywall!
thanks again for the advice, everyone.