Unfortunately, 3D printer materials are generally not very strong, and have very poor dimensional stability at warmer temperatures. A 3D printed version of most vacuum parts would not last long, I suspect. That said, some things can be printed as fully functional parts, and even if it isn't good for a long term part, quick prototyping with a 3D printer is amazing. And making that available to more people, and cheaper, is great.
I think 3D print materials are something like $0.30/gram (and a little less for the support material which gets thrown out); add to that printer time and it could be a while before distributing parts via 3D printer can compete with, say, injection molding if you are distributing significant quantities of the same part.
I suppose printed parts will get better with time. There are companies out there that make very good parts with SLS quickly (http://en.wikipedia.org/wiki/Selective_laser_sintering); you can even SLS titanium.