We used to have a client that used them on her GSD mix and Rottweiler mix - the Halti kind.
I'm pretty sure she didn't go the extra mile and "properly" introduce them. She was by no means into dog training at all.
Her dogs would actually put their noses in the hole and were excited to have them on. Personally, I think it's like teaching a puppy to walk on a leash and wear a collar. They aren't wild about it at first, but if you make them wear it ALL the time, associate fun stuff with it, etc. I think they learn to accept it but I also think they have to wear it a lot in order to do so.
Personally I think the harnesses and head halters are worthless just in the sense that they don't really teach the dogs anything. Yeah so the dog doesn't pull as hard with the halti on, but when you go back to a flat collar they are back to pulling. I think the two leash idea would be good because with the use of the Halti you could teach them the feel of loose leash walking with a leash attached to their collar.
I think it's definitely harder to introduce it to an adult dog, but again, I think it's all about making them wear it and live with it. We had a client who had a dog that had seizures really bad and the dog kind of lost its mind after a really big one. The dog was a 5 y/o schnauzer mix, the woman had some mental issues [so again, not a dog trainer] and she made the dog wear a muzzle when they were out walking and the dog accepted it no problem. However, I think the dog was just forced to wear it and deal with it and the dog figured out it could wear it and still go for walks and still function with it on.