I have had a number of frames custom built for me over the years, including a few by some very well-known boutique builders. I have come to the following conclusions about the topic:
- a custom geometry is only worthwhile if it works well. I have seen many a custom geometry frame that may fit the rider on paper, but is thoroughly unrideable in practice. So don't go to a framebuilder who doesn't have a good understanding of geometry. A proven geometry requiring minimal modifications of the rider contact points is far better than an unproven geometry which gets the rider contact points right.
- A custom frame does not simply involve modification of tube lengths. It should also foresee an appropriate selection of the tubing (weight, diameter... etc), fork (rake and trail) and wheelbase as required by the individual rider. If the "custom" framebuilder does not take your weight and riding style into account, you are not getting a real "custom" frame.
- A custom frame should fulfill all your aesthetic priorities and desired braze-ons and accessories.
Having said all of the above, one should also take price and delivery timing into consideration. Indeed, you can often find a custom framebuilder who will build you a custom frame for less than many much more mundane frames, or industrially-produced frame that carry a fancy name.
I recently arranged for a friend to have a custom frame built for him by Daniele Marnati in Milan. When Daniele came up with the proposed geometry, my friend was totally convinced that the frame was the wrong size for him. He nonetheless put his faith in Daniele and me and took delivery of the bike. It is now by far his favourite bike...