Strange, I've rarely heard of them staying on the backing, haha. The ones I had worked pretty good on a clean (clean ONLY - any dust and the brake was terrible) Mavic CD rim. Wouldn't like to try them on a grind though, pretty sure they wouldn't last long. As for the 50mm versions, I once disassembled a set to nick the backings - I had the fright of my life when I saw they weren't even recessed 1mm!!
Rob: Grind your rim, set the brakes up, and leave them. You don't need uber-fresh grinds with CRMs :bunny: Every time you re-set the brake up, it has to bed in, and this destroys pads unless theyre perfectly square (impossible really).