A few misconceptions about singlespeeds floating around here, in my opinion anyway ( assuming that we're talking about mtb's and not track bikes, urban fixies or that sort of stuff).......
They're only any good on flat terrain - no, singlespeeds are at their
worst where it's flat - give me hills (up or down) and that's where a singlespeed comes alive.
They'll ruin your knees - no, they won't - not if you don't try and push big gears or climb in the saddle. Any time that the trail starts climbing you should be out of the saddle and that's a lot easier on your knees and back than spending loads of time sitting down.
You need to spend big money on a custom frame - no, you don't.
Go to somebody like Singular, Genesis, Sanderson or, dare I say it, On-One and see what they do.
You'll have gathered that I like singlespeeds - I like the simplicity, the fact that they become part of me like no geared bike does to the same extent and the fact that, apparently, I ride them far better than I ride geared bikes. Unless my friends all lie to me to keep me sweet, of course
I like the smoothness, the silence, the fact that in technical going you get a "sixth sense" of exactly how far one pedal stroke will take you - it's more like walking and running, in that respect.
No mech. to collect debris, no thinking "I could have cleaned that section if only I'd been in a lower/higher gear".
I have no doubt that, on these forums, there are people who are far better exponents of the art of riding a singlespeed than I am and I don't claim to be the fastest/strongest/fittest but, for an old fart like me, there is something about them (SS's) that is strangely encouraging........
Encouragement to get up out of the saddle at every opportunity, whether it's tackling a short, steep broogh or a two mile long sheep track up a heather-covered hillside, to use momentum well and to hold speed on every bit of downhill (because you can't pedal to get it back) and just to loon around like I remember doing fifty years ago on some old bike that my Dad had just fettled for me. Except with better brakes.
They're not for everyone, I accept that, but if I could only have one bike then it would have to be a rigid, singlespeed 69er.
It would more than likely be a Singular Hummingbird.