I built my Singlespeed out of an old low range GT, I built it as a cheap no frills pub/shop/errand/doss about bike, The intention was to keep it cheap and simple, but no thanks to this place it has evolved to the point where it is now my favourite bike and has nice wheels, brakes, bar/stem combo, etc, etc.
I dont think i'd take it off road as i have my FSR for that, But for bombing around town and just the ease of riding without having to think its perfect
Another benefit to singlespeed is it got my mrs into biking, She loathes biking after trying it once years ago, but when i built mine she liked the simplicity of it (she is female)
so i built her one and she loves it, She has a tiny little stumpjumper again built on the cheap from Retrobike parts.
Singlespeed rocks, and if i had the money i'd love to build a nice bespoke one just for the posing factor