No I have not used one before.
I found out about them from a presentation at our U/W photo club. The presenter has used them.
Back ground info: I ran a large gear manufacturing company for three years and do have some insight into gearing.
Think about how much pressure you would have to exert to deform a plastic gear tooth and think about how much force is requires to turn a lens ring.
If interested, pm me and I'll put you in touch with the guy who presented at our club.