I find it fascinating ,,, my son built his 3D printer and is now using it to print some of the pieces of a CNC 3D router he is now building. It's amazing what things can be produced and how robust they are when the density is set high. He's made some replacement cogs for his milling machine when the original nylon one broke .. new replacement ones are about £9 a throw, the 3D printed ones he's work out about 30p .... he's printed enough to replace them every month for a year but 6 months in he's only had to replace it once.
It's all above my technical ability and I marvel at what can be made.