A matter of craft
I think the difference between a custom hand made bike frame and an off-the-rack frame is like the difference between an off-the-rack suit and a tailored suit. Not only can a frame builder take into account your height, but also your proportions, inseam, weight, build and how you ride. Plus, of course, what you wish to spend is a major factor. We are all built different but off-the-rack bikes often offer generic off-the-rack sizings.
Plus, there is the art of it. With a custom bike you are riding a living, moving piece of hand crafted art. You are supporting real artisans. The personality of the builder is in the bike. He/she hopefully has expressed that. How he/she thinks you will use the bike is reflected in everything from the length of the tubes to the types of materials and the construction methods used in the bike.
Joe Starack built my first custom bicycle, a Rivendell, and Joe Bell painted it. Local builder Dale Saso built my other custom bike. That bike was the result of visits to Dale’s shop, of long conversations about types of tubes, materials and riding styles. I remember holding and looking at bare steel tubes and going through boxes of fork crowns and being able to specify the most small of details on that bike. It was a collaboration effort. I saw the bike evolve from metal. My Saso bike is the result of the many conversations Dale and I had. Does that make me faster? No. Does that enhance my own personal riding experience? Yes, very much.
You may be surprised to find the cost is not as high as you fear. Only you can decide what you want. That’s what makes’em so cool.