gentle leaders are worthless for giving corrections. But, i have used them before, and out of all my dogs, i would only ever use them on one. And that is for pulling, because the dog would literally pull so hard with the prong that it was bleeding. She instantly stopped with the gentle leader.
Now, as far as the pulling the head away for aggression, that doesn't work.
So, with a few dogs who are strictly pullers, not animal/human/dog aggressive, they work alright. I have seen several dogs, including one of my own, walk very nicely in a matter on minutes.
I have also seen too many still lunging at the end of that gentle leader, with no fur on their nose.
So with my dogs, and this is just my experience, with Maya, she shut down on those, she went into a panic, would drool, the whole works. I put a prong on her, and within seconds, she got it, she had to listen.
Bart wore one for about a year. It worked great, at first. Then he learned to pull and lung while wearing it, he would put his head and neck at a certain angle. Bear in mind, i tried a prong at first, he would not have it, it over stimulated him, plus, when he wanted to go, no amount of popping would stop that, and the prongs were causing marks. So it was gentle leader for a while till he learned to listen. Now he can wear a prong, and not hurt himself.
Roxie can or can not wear a gentle leader, it depends of the day. With a G.L she doesn't pull, but she lunges as squirrels and cats. With a prong, she doesn't lung, but she pulls. So i can switch back and forth, or she can wear both.
That is just with my dogs, and i don't use them to give corrections, as they aren't designed to to that.
I know many people who use them, and most of them don't know why, or are using them for "training". I see dog aggressive dogs lunging and snarling with these on, and the owner is clueless because their magical gentle leader was supposed to fix all their dogs behavioral problems, along with treats.
I have seen dogs pull, and hurt them selves on these things, because the owner wouldn't use anything else, after all, they are GENTLE Leaders, and can't possibly hurt the dog, right?????
And of course everyones favorite scene, someones dog DRAGGING them to the dog park, all hyper and already acting stupid, and of course, they are wearing a gentle leader.
So, for me, and know i will catch crap about this,
some dogs can use them, and do very well, but that is a select few.
The vase majority of dogs wearing them need something else, namely, they need a prong and a different owner.
My trainer recommends a gentle leader at first. But the second she sees that it's not working, she doesn't hesitate to use a prong or remote collar, and she knows how to use them correctly. She just gives the pups a chance the "gentle" way.
For dogs with issues, and more high strung older pups, she goes right for the prong.
So in all, i think that they are pretty much worthless for maybe 82% of the dogs wearing them. the rest do well on them, for now at least.