I like vintage mtb's for lock out bikes or beater duty. For actual mountain biking however they pretty much suck compared to newer designs. In my opinion only.
Remembering when I first started mountain biking trying to go fast sometimes meant that you would be bouncing so violently that you could not see after a point. with modern mtb's and full suspension I have found myself going terrifyingly fast a few times and almost able to stay seated! Old mountain bikes make good tourers too.