I've got a cheap 3D printer (
MP Select Mini 3D Printer V2, Black - Monoprice.com). It's the only one I've ever messed around with, but from what I've read, it works as well as some others that cost 2x or 3x as much. It's the kind that works by squirting the melted plastic filament into layers, like squeezing cake frosting out of a decorating bag. There's another kind that uses a special liquid resin in a tank. It projects an image into the tank with light which solidifies the resin, and it builds up layers like that.
Anyway, I've used it to make replacement plastic pieces for broken things around the house. One thing to keep in mind is the orientation of the part as it's printed out. It's much less strong between layers than it is within a layer, so if the piece needs to be strong in a particular direction, you need to plan ahead. That also means that the replacement pieces I've printed out almost never look exactly the same as the part it's replacing. I'll dig out the calipers and measure the important dimensions on the old piece that can't change, and then use a 3D modeling program to design a new part, taking into consideration the how the 3D printer plastic behaves differently than whatever the old part was made of. Probably by adding supports, or making some parts thicker. And then I usually have to go through a few iterations before the new part works well. A 3D scanner might give you a jump start on that redesign, but it'll still need some tweaking.
Because of the way it builds up layers, you can see them in the finished part. Do an image search for "3D printer layers" to see what I mean. You can sand, use body filler, paint, chemical treatment, etc. to make it look better. They're not (yet) like magic replicators on Star Trek, just another tool to learn how to use and when (not) to use it.
