I have a Gentle Leader at home. I bought it about two years ago when I ran into a pulling issue with Gaci and I wanted something I could use at those times when I didn't have time to teach LLW. I spent three weeks getting her acclimated to it. Not wearing it full-time, just getting it ON her and having her wear it for short periods. Even with three weeks of slow, methodical work, one day she up and took off when I brought it out. She ran into my bedroom, crawled onto my pillow, and laid down. She wouldn't budge, with any amount of coaxing. I tried the next day, same thing. She was absolutely, totally, upset at the idea of putting that collar on. Even though going on walkies is one of her FAVORITE things to do, she still ran and hid.
Because of my use of the Gentle Leader with her, she now is totally uncomfortable in "getting dressed" to go on walks or drives, with any equipment. Because she's got a normal martingale collar, her tracking harness, her front-clip no-pull harness (I got it after setting aside the GL), and her seatbelt vest, it is hard to watch her unhappy about putting these things on. I have worked with her on these issues for a couple of years now, and while she now tolerates it, she still doesn't like it, and you can see when you put her collar/harness on that she shrinks just a little, or puts her ears back, to acknowledge that she's still not trusting of putting new things on. The difference now is that once she's dressed, she totally forgets about what she's wearing. It's not the wearing that is the problem, it's the "getting it on and off", a memory from when she used to wear the GL.
She also didn't take well to the GL, as some people describe. She turned much more subdued behaviourally, but on the other hand always appeared to be much more on edge and anxious. It was a strange contradiction. She wasn't enjoying her walks, she was simply tolerating them. And I think that's why she finally decided that enough was enough, the walk wasn't worth that feeling for her.
I've since set it aside, but I do keep it in case I someday find a use for it. But for my sized dogs, I have found that a no-pull (front-clip, not restrictive) harness does the job for pulling equally well, and without the negative side effects that occur in a lot of dogs. And there is virtually no conditioning required for these types of harness, you just put 'em on and walk.
I do see the effects in dogs that people here are describing, even when they are applied properly. There are very few dogs who "like" the GL. Some dogs do learn to tolerate it relatively okay, and some dogs don't care much, but for the most part most dogs don't really like them. I've experienced this on some of my training lists as well, a lot of others seem to notice this as well. What was once considered to be an ideal "pain-free" tool to use, now isn't as recommended by some as it used to be. Others still swear by it, but I wonder if it's because of the usefulness or because of the problem in admitting what you recommended for so long isn't as "positive" as it was once called. To me, the dog determines whether or not it is positive, not the end result of ending pulling, even if it doesn't apply pain. For a lot of dogs it has nothing to do with pain, but it's still not at all a positive experience for them.
There are a few dogs that I would consider using it on rather than a front-clip harness, that is if the dog will tolerate them well (I couldn't use it on dogs that it adversely affected, no matter how useful it might be). Dogs that don't fit into these harnesses well, because of chest depth or other issues, sometimes work better on a GL, if they tolerate it. But you can get a wide variety of front-clip harnesses now and I think there is a kind that will fit most dogs now. I might recommend them for over-the-top exuberant dogs, because the GL does have a subduing effect on most canines, and it might just be the speck needed to calm that hyper dog enough to listen a little bit.
I would no longer, ever, recommend it in cases of shy/fearful/underconfident dogs. These dogs need a confidence boost, or to feel less anxious, not to have something on them that causes them to be more upset or subdued. I wouldn't recommend them to be used on lungers - dogs that don't necessarily pull, but that have a history of lunging unpredictably (or even predictably for that matter). The companies mention that there are no reported cases of vertebrae or muscle damage with these items, but I don't believe that for an instant. I can't see how a dog lunging quickly, having it's neck snapped sideways and downward, while the rest of its body is moving in a forward momentum, is not risky for a dog's head and neck. I've seen some of these dogs' heads do this when they lunge ,and even if the dog only has a half a foot or foot of leash, they can still get that momentum for a lunge even though they don't get far.
Overall though, for simple pulling cases, I think in terms of "tools" that a front-clip harness, will suit most people's needs for pulling dogs. I know I wont' automatically recommend a GL for a puller, and I'll try to find an alternative if possible (unless the dog seems to be the right candidate for one). The GL really is one of those things I'd use on a case-by-case basis.
And yes, in the end the GL, or Easywalk harness, or even choke chains and prongs, or whatever you use, are meant to be used as training tools only. That is the mantra that is chanted these days. That they are "not life management strategies", although many people relegate their dogs to all of these types of tools as life management. And you know, to me that's okay, because they don't want to take the time to teach the dog, or don't have the physical ability to teach the dog to walk nicely (frail older people, people with disabilities, smaller-built people). I'm one of those people who is not "against" using these items routinely on dogs (well, I would never use or advocate prongs or chokes, personally, but to each his own), IF the dog is tolerating it well and enjoys its walks. It's not ideal, but the world is not ideal, and if the harness keeps a dog in its forever home because the people cannot, or will not, take the time to teach it on a flat collar, that's fine with me, because that's one less dog that is likely to face a needle if we pushed "teaching LLW' on people who just can't or won't do it. So no, I don't care if people use these tools for life, if the dogs' quality of life during wearing it is still high. In a lot of cases these simple "tools", as we like to call them, are reaslistically life-savers for many dogs who otherwise would be given up or abandoned, or worse - relegated to a life short of natural exercise, where in turn they will develop many other behavioural problems that will lead to again, being sent to a shelter.
For me, it's a management tool, used only in times I cannot teach. But that's me, because I have faith that I can,and will, teach exactly what I want my dogs to do. Not everyone else has the interest, or the knowledge, or the time, to teach as I do, and for those people they need "tools" to make living with their animals a little bit easier.