Hi I just started using a GL on my younger dog even though I've always sworn I hated head halters. I hated them b/c I saw too many people use them improperly (too loose and/or the dog straining and bucking against them) and become dependent on them.
My younger dog is quite stubborn and head strong. He is VERY social and anytime there is a new dog or person, he wants nothing more than to meet them. He is a very sweet dog, but he doesn't seem to *get* that it's just not appropriate to always be socializing. He is in a beginner agility class, mainly so we can work on this issue. The first day was a mess because he was on a flat collar, pulling me all over the place. This dog basically lived on a tie-out before we adopted him, so anytime he is on a flat collar, he pulls and pulls.
Since this is an agility class, we are limited in what tools are safe, but I noticed someone else using a Gentle Leader and got permission from the instructor to try it. When we first got it, we kept it on him only a few minutes at a time, and gave him nonstop treats and praise. Now, we can take him on a long walk with limited treats. I do not want to become dependent on it, so I alternate walks using it and using a flat collar. Last walk on the flat, he was a little naughty on the way to the park, but on the way home he was a total angel! I think letting him get a lot of exercise before training his leash walking also helps.
I am happy to say that he has never flipped out, bucked, or tossed his head while wearing the Gentle Leader. The GL = fun agility class, and after class, I take off all his collars and let him play with the dogs, so ultimately he will get what he wants, but I get what *I* want first.
Fit is crucial, but Bonnie has done a great job explaining that.