It has taken a while for the printers to come down the cost curve, and for materials to come up the quality curve. I was talking to the CEO of Z Corp (a $50M 3D printng company) and he gave a great example of how, for Clark's Shoes, they've been able to go from small scale looks-like prototypes, to full-size prototypes, to functional shoes, over 10 years. Apparently functional prosthetics are next.