I have thought about all this quite a bit as a retailer, builder (occasionally) and a appreciator of vintage bikes. Serotta and Waterford make very very nice bicycles. CoMotion, Davidson, and maybe some others can be added to this category, in that they are semi-production handmades. I sell Waterford now in cycles de ORO and have sold most of the others. They are more custom than an Italian bike but certainly not true custom. A true custom would be made one at a time, specifically for one person with all the possible variables .
I think the problem (not much though) I have with these semi customs is that you can buy a truly custom frame from a number of sources for the same bucks. And have all the benefits therein. AND have a frame in which, for instance, all the seams and casting ridges are filed, the top tube length, frame angles, tubing gauges and diameters are all unique to your needs... And you can often meet and talk to the actual guy who does all the work..
I guess it is a lot more work to find a builder.. figure out what you want and then wait for it to be made.. But the return benefit is that you have a bike that not only fits you and rides the way you want to ride, but you have a piece of craftwork that is above the ordinary...
Dale Brown cycles de ORO, Inc. 1410 Mill Street Greensboro, North Carolina USA 27408 336-274-5959 Fax 336-274-6360 <A HREF="http://www.cyclesdeoro.com">cyclesdeORO.com</A> <A HREF="http://www.classicrendezvous.com/main.htm">Classic Rendezvous</A>