Are you talking about buying a mountain bike for mountain biking (i.e., riding singletrack), or for getting a more comfortable position on the road? I ride with a friend with some herniated disk problems, and even his FS bike bothers him on rocky descents/drop-offs. If you're looking for a more comfortable road position, you really should test out some recumbent or semi-recumbent bikes -- apparently some people find amazing relief from them. My Dad is riding a semi-recumbent trike after his knee surgery, and loves it.