There are a couple of threads on this forum about them; with some folks intelligenting arguing that they can be used non-coercively and non-abusively in certain limited situations, that they are a tool like any other and whose severity depends on the skill of the user.
AND I will confess, that while I am usually anti gadget and pro slow, systemic training, I did use them once, briefly and fairly successfully, on a very difficult OTTB for whom all the traditional training techniques I tried did not work. I lunged it them to help us get past the horse's initial resistance to contact and working on the aids.
However, they are most often misused, for all the reasons stated above. There's a catch to them, like most training gadgets: If you're tactful enough to use them without being abusive, ie, to release pressure as soon as the horse gives, then you're tactful enough to do the job without them. If you *need* them to get the horse to give; you're probably abusing them.