The very first 3D printed objects I saw were flimsy plastic parts. These were great for prototype cases for electronics, but you probably wouldn’t want your health and safety relying on them. This excluded many applications related to machinery and transportation. This now appears to be changing as 3D printing with metal is gaining momentum. See these articles below:

Daimler Funds 3D Printer for Auto Production

Ford Builds Metal Prototypes With 3D Printing

3D Printing with Metal, Titanium & Aluminum Demo by EOS @ MDM 2013 (YouTube)

Metal Printing Summary from ExtremTech.com

Hopefully this will be a trend and we’ll see fewer plastic trinkets and more heavy metal.

Oh, and speaking of “rocking”, check out this article about “3D Printing Giving Life to Old Recordings” about printing copies of some of the very oldest recordings
or this one about the microphone for the U2 tour being 3D printed!