The Deja Vu thread on singlespeeds.
I think everyone in our little riding group (once known as Peak Midweek, but no more) have had a flirtation with singlespeed, and some have found it so much fun that they have stuck with it. In my case it the flirtation has lasted, erm, 8 years, I rarely ride a geared mtb.
Whether you're fast or slow, can ride big hills or rocky descents is more down to the rider that the number of gears. Singlespeeds are generally slower than geared bikes, however, singlespeed riders (especially in hilly places) are often quicker than your average geared rider, it's a well known fact.