I have had a mk3 for a couple of years now and carnt fault them(excellent bit of kit)but i prefare the older grey collars that you open and shut to turn on and off to the magnetic switch ones.there more slim lined around the ferrets neck and i have heard of more people having problems with the magnetic ones.I have recently swoped two of my magnetic ones for two of my mates grey ones so im well happy
I would only reccomend the use of e collers as a last resort as bobslurchers said with very difficult dogs.incorrect or over use can ruin a good dog,there is no substitute for perseverance and being consistant.