I know the bear hunters up in northern Wisconsin use them to deter wolves.I never heard of someone using them for coyotes.
i have bear hunted twice this year in northern wisconsin and have seen a couple guys that have bells on their hounds. like toe cutter says DEFINATELY a dinner bell. im not saying they wouldnt work right away, but either yotes or wolves after awhile figure out the sound of bells could be a free meal.
our group of dogs got surrounded by wolves and four of the six had bells. we luckily only had a couple get tore up before we got to them. we just happened to be close enough.