All I know is what I've scanned across on the web.
The printing is in plastic, from what I gather.

Most of the press about them concerns guns. I'm sure there are other uses, but without the political agenda.
Last article I saw said something about sending one to the space station so they can make stuff.

When they can print out of recycled material, economically, will likely see a lot more info.

If I had one, I'd print, for my modern bikes:
stem/handlebars customized for my hands, a couple with integrated lights
saddles with integrated seatpost and wedge bag, customized for my bum, with an integrated tail light
cycling shoes custom-fit to my feat with built-in cleats, but walkable
cycling helmets custom-fit to my noggin with built-in eyewear (for Rx lenses)
custom water bottle cages, though I like what's already available

Of course, almost none of it would go with my steel bikes. For those:
a custom saddle "base" to work with my existing rails, to be leather-covered as well.
helmet as above
shoes as above