Just finished a Barbershop.... and very pleased with it, too. I use it as a low gain overdrive. I'd have another as an always on pedal if I didn't already have a ROG Ginger, which I use as an always on, barely detectable grit pedal. The Barbershop sounds good into the Ginger, and an LPB-1 works nicely to give it an extra push when needed.
The black control knob is just a 'dummy' to fill a hole. I'd drilled the box for an earlier project which I didn't like.