I have seen one of these, and even tried it - it's more typically called a "chi" machine, meaning the Chinese name for intrinsic energy in the body.  I have not read any medical studies, but I would be cautious.  I think it could conceivably help with lower body circulation, and maybe the vibration would help a stiff back or hip - but why not check with your doctor as to whether it would be safe for your joints? 

     However, I don't personally see how it could subtitute for actually using the muscles and tendons, since it is a passive device.  If it makes you feel better or more relaxed - great!  Is it worth the $$$ you'd be paying?  That depends on you - I have seen them advertised for around $600.  Have you tried one?  I would also be cautious if you have foot or ankle problems, it might aggravate just by friction.  But the joints need to be used to stay flexible and strong.  No passive machine will do that for you.