If I'm not mistaken, many if not all of items Shapeways sells, are developed by third parties, including potential customers, and then printed by Shapeways. For example, you can use a computer-assisted design program to create a file for use as a pattern, and send it to Shapeways to print it. I think they then add it to their catalog as well, but I have not used them, though, so I can't speak more closely at how you proceed with them.
Their quality can be quite good. A friend of mine in our club has ordered a lot of their "castings", especially the dinosaur models that they sell. They are nearly as good as the far more expensive solid resin models produced by sculptors and cast in a garage industry, and much, much cheaper.
As Ed mentioned, the process still isn't perfect. Much 3D printing is done by extrusion, that is, with a nozzle that moves along three axes, directed by the software pattern, laying down a bead of the material. MakerBot's 3D printer originally used that technology, which was adapted from dentistry, by the way, used to make dental prosthetics. Their early machine produced objects with definite ribbing, following the line of the extruder as it made a pass and laid down a bead of the material. I think it's gotten better, in the meantime. But if you do a web search on "makerbot", you might still find the series of videos they had loaded on YouTube as the user manual for their v2.0 3D printer.
I think we're getting ever closer to a point where the technology becomes cheap enough that many of us might have a 3D printer on our benches, and instead of buying a kit, we buy the pattern from the model company, and print the parts ourselves.