I've marvelled at the fact that my Summicron which was is 51 years old still feels amazingly precise with no slop in the movements at all. It's certainly built to last. Solid metal construction throughout. The only plastic is the little red dot for aligning the lens during mounting!
Very neat video. I'm surprised by how much hand assembly they do for their lenses! There was another video posted a while back that showed how Canon manufactures their 500mm f/4L lens... here is the link.