On a physical basis, the halter is probably the one piece of training equipment that appalls me most - the potential for injuring the dog is simply too high. I'm not talking about snapping the dog's neck or crushing his trachea - I'm talking about soft tissue damage and damage to the spine, particularly the cervicals. At numerous APDT conferences, I've had the opportunity to spend entire days watching trainers and their dogs. Many of these dogs wore head halters, not surprising since APDT attracts many trainers who are interested in humane and positive approaches to training; the head halter is seen as both. What horrified me was the number of people (remember, these are professional trainers and serious dog folks!) who would simply stop at a booth, allowing the dog to drift ahead until he reached the end of the lead and then had his head brought sharply to one side. Watching this repeated over and over again, I began to feel that I was watching people casually moving boats in water - as if the leverage and force made possible by the head halter had little more impact to the object on the end of the lead than a canoe might experience!
NOTHING in the dog's physical construction or his nervous system prepares him for the force of an unexpected, externally directed, sideways and upward movement of the head while his body is still moving forward (sometimes at considerable speed!). For the horse, the leverage is similar but with key differences: the force is directed sideways and downward, and the muscles of the horse's neck are among the most powerful in his body. There is also a considerable difference in force that can be applied to a 1000 lbs. of horse vs. 25-75 lbs. of dog. Interestingly, when working with young horses, ponies and miniature horses, care must be taken in the use of the halter with allowances made for the height difference - knowledgeable handlers do not apply force upwards and sideways, but turn the animal's head in the same plane as would happen with a larger horse.
That's one of the things that bothers me about them. If one of my guys saw a squirrel and bolted, that could be bad.
I tried one on Simon for a couple of walks. He was miserable. And for a dog who was already anxious when out of his element, sensitive, fear aggressive, adding to his stress was no bueno. No, I didn't introduce it as slowly or properly as I should have. I tried it on Robin, just to see how she'd do, and she was ok with it-- not overly upset, but not happy about it. But if I've got a dog that's already scared or sensitive, I'm not sure I want to add anything that might be uncomfortable or in any way aversive in a situation that I'm trying to convince him is the opposite if I have other tools at my disposal. Just my 2cents.