I've been using them for at least 15 years, possibly a bit longer. I've gotten mine by swapping material, hooks or flies for them. Use them mostly in fresh water for trout, pan fish and bass. A little leery of using the ones made from tying thread in salt water. I have a couple made from fluorocarbon that I use in salt water.
As Jack said they last forever and they turn over flies ranging from size 26 to 1/0 without a problem. Once I started using them I never went back to the hand tied mono or fluorocarbon leaders,