best folding bike shop in San Francisco ?
I can recommend Warm Planet Bikes by the train station at 4th and Townsend. They have a small collection of Dahon, Bike Friday, Downtube and Strida bikes there.
One of the guys working there in the morning is pretty knowledgeable and communicative. Just don't go there between 7am-9am because the store also does business as an indoor bike parking lot and they get swamped with train commuters (the people that don't have folders) checking their bikes in or out. After the rush the staff have lots of time to talk. (probably another rush in the late afternoon too).