I have used Jez's knob for about 4 years now and I'm convinced it has saved my board from mortal damage on many occasions. There are some issues with positioning the mast whilst treading water for a waterstart but some practice and thought present solutions.
I lost my Jez's knob last week in heavy waves and strong winds. It seemed to break out of the thread of the nut that holds it into place while doing the aformentioned mast positioning. Stange, I thought, but no arguing that this is what happened. I'm picking another one up ASAP.
To date I've received no damage to any other part of the board either mast track or surrounds other than some scuff marks on the deck from the plastic flange that comes with the knob.
Jezza. You've got a good Knob!!
Simon.