I recently commissioned some hardware from Shapeways. I needed some miniature brass castings for my campaign piece. The process has been slow and a little expensive as I don't have 3D drawing tools or the ability to get them at present. I needed to have someone do the drawings for me then I uploaded them to Shapeways.
From my understanding to get the brass parts they print in wax and then do a traditional "lost wax" casting in that the wax master is embedded in a slurry of plaster the wax is burned out and then the molten brass poured in. So the 3D printing is actually a small part of a very traditional process. My first pieces have actually now been printed, cast, and are in the mail so I will report back once I receive them
I was going to try forming the pieces from sheet brass but real campaign furniture usually used cast hardware and I try to follow the prototype process as much as possible. I tend to agree with Mike somewhat in that a printed plastic chair has no meaning to me it is lifeless. But I don't see that it will suck the life out of the hobby so much as fill a need for the low end entry level product.
So to me i see this as a step in part of a whole process to make some of the parts but not to replace the hand creative process. I also had a block of shotgun receivers printed in metal to use in making some 1:12 scale classic double shotguns. The stocks will be burl walnut carved from blocks and some of the parts, like the trigger guard, cut from .005 brass sheet, but the receiver itself is a cast of a wax printing. So again I see this as a part of the process. Printing the entire gun would be, to me, plastic and cheap but using the technology to make components to enhance the had work I am ok with