I built the Timbuk2.com bag customizer (http://timbuk2.com/customizer) and I would say that 50% of their business is on the custom side. People are playing with the product and often they surprise us. Timbuk2 ends up charging a premium for the custom bag, but it a unique bag.

Same applies to Nikeid or adidas or converse or ... A lot of people are trying to build custom products and let the consumer get access to their manufacture, but it is not as simple as it sounds. There are, as in all things, a lot of moving pieces and enough rope to hang yourself.