heres my 10p's worth.
having had alot of different bikes over the years, and ridden many more on top, i think i know what i like a bike to 'ride' like. if i was ever to consider having a custom frame built i think i would go with the massive knowledge that the bicycle industry has already accumulated and choose relatively standard geometry but have a frame with particular features, like a sloping top-tube, lack of chainstay bridge, under bb gear cable routing and under top-tube brake cable routing, the little touches that i have learned make a difference to my riding. i was considering getting a custom ti frame from canada about 4 years ago, but in the end decided to get an off the peg bike from one of the big u.s. builders instead because it offered all of the features i wanted at the time, but also a better warranty and the safety of good geometry and fabrication that i would be proud of, and at the time it was the same cost as the custom frame. i have been happy ever since.
oh, and the custom frame that resides in hereford with the dodgy paint actually rides really well, climbs well, jumps well, and is very stable over rough stuff, even if the owner doesnt like the head angle, i do. i think he wanted it too slack and the builder may have done him a favour.