I have tried 3D printing for some plastic prototypes of certain tools before making final decisions on designs. When using sites such as 3D hubs, I found that many of the companies
that are selling the use of their 3D printers are marketing them as just that: Great ways to prototype your ideas.
The guy I talked to at the company said that the majority of their business came from people who had designed some sort of devise, or had had commissioned them to design a circuit board, and then needed
a casing to form the final product. They seemed to be happy using their 3D printer for that purpose only. Not really expanding the horizons of 3D printing but using whatever existing technology was out
to cater to people who want to quickly see their vision of a product, and iterate there forward.
From what I observed, I can visualize that this and similar applications are where 3D printing will take root for a good while. So it is already successful, the ease of access is great for the end user, but I don't see it growing away from such a market without a large leap in cost efficiency.