Yes Lorelei - I've used them :) (MIKKI dog discs) they should come with a booklet and you habituate the dog to them
before trying to use them to stop him chasing etc. The Booklet explained things very clearly and was easy to use :)
The length of time it takes for the dog to understand what they mean varies with the dog, my dog cottoned on really quickly(matter of minutes) but my B-in-law's Sheltie took days to "get it" Once they understand the 'message' the discs are merely moved in the hand and are very effective.
I understand what you are talking about, my Bedlington has a very high prey drive and would also probably be dead (along with a few cats) if I hadn't used these
Good luck with this, it can be so demoralising when all your training seems to out the window at the mere smell/sight of a cat or fox.