Okay i'll throw my opinion in just for kicks.
Like many I rode and enjoyed a gear bike for many years, it's easy and fun. See a hill, dump a bunch of gears, sit and spin to the top. When the trail goes flat shift again and hit warp drive. One of the added advantages (of my full sus) bike was that I could also plow through the rocks and keep my speed up. It was great fun going as fast as possible all the time until one day I realized how lazy I have become as a rider, choosing bad lines because my bike allowed me to float over and through, always hunting for the proper cadence.
A few of my riding pals (single speeders and bike mechanics) ride only single speeds, so I thought i'd give it a go.
I proceeded to build a nice steel full rigid single speed and it really opened my eyes. First it was pure misery standing and grinding up hills, feeling all the bumps and getting pounded by the rocks. Then one day it just clicked, yes it still hurts but on sustained climbs i'm far faster than before, my handling skills have improved ten-fold, and i'm almost always quicker overall then my geared pals because I go faster uphill and smoother everywhere else (with the exception of our local babyhead gardens). Plus it is always great fun passing geared riders on a ss, both going up and down.
I tried to ride my squishy bike again and just hated it, too soft, too much going on.
Not sure if i'll ever go back to gears, for now i'm super happy on my 1 geared wonder.
Oh and yeah, I ride mountains.