I learned about the use of headcollars while I was studying my program, I was intrigued by the idea of the product and had to run out and get some to try. I brought both of my dogs with to fit them with the headcollars at Mare-Z-Doats. We were immediately greeted by a gentleman who helped us figure out the right size for our different sized dogs, and he showed us how the headcollars should fit. It was very helpful because when you first look at the construction of the headcollar it can be a little bit like 'what goes where?' We only had the dogs wear them for a short off leash time the first few wears as it is definitely a strange feeling for the dogs at first, especially if they are adults. You can expect to see a goofy reaction the first few times you put a headcollar on your dog, similar to a wet dog who rubs his face all over the floor. (This is an extremely normal reaction as you can imagine it would be if you started to wear a tight mask on your face or unusually heavy makeup.) If you start your gentle leader training with a puppy this will be a much less dramatic process for your dog. It is the same process as getting a puppy used to wearing any sort of collar, and eventually they will accept it and you will reap great rewards.
My dogs have been using their gentle leaders for 2 years now. Rico still does not enjoy having his gentle leader on when he's stationary (for the 2 minutes he has it on before our walk starts) but he also gets excited to see that I'm grabbing it because he knows it means "WALK" and he starts jumping with anticipation. As for Momo she is now 100% accepting of it.
Let me now take a moment to explain why I love this product so much. Dogs are not perfect. While extensive training can give you a dog who starts walking when you do, walks by your side, and stops/sits along side you, the gentle leader gives you the extra help you may need to control your dog before you may reach that point. Not only does it give you control over a large strong dog who pulls, it also gives you the ability to control your dog's mouth (only if you should need you).  The gentle leader is commonly confused with a muzzle which it is far from. Your dog should have complete use of it's mouth, be able to open wide, drink water, fetch a ball, and pant. But should you need to close your dogs mouth to avoid it barking at a stranger or trying to fight another dog you will have complete control.  The mechanism of the gentle leader is a sliding loop around the snout and additional collar around their high neck with one single attachment, if the snout loop should come off the leash shall still remain attached to the collar and your dog can not slip the whole collar (as long as it's fitted properly). To understand how the gentle leader helps prevent pulling vs. a chain collar understand this; a dog holds a lot of strength in it's neck and if your dog really wants to move forward with a collar around its neck whether it's tightening or not, it will move. If you have control of your dogs head it will swivel on it's neck giving you the control.  
Even though my dogs are not very large, they USED to pull ahead of us. Now I walk with Rico on a 1ft training tab and I hold it loosley with my index finger. He stays right with me, and if he tries to stray from my lead the gentle leader directs his head back to me. Rico also suffers from fear aggression of strange dogs so on our walks I need to have complete control over him to prevent any potential outlashing, the gentle leader helps as I can put him in a sit and hold his mouth closed with the mechanics of the product.
I highly recommend this product, as well as a high recommendation to request help when you first purchase to avoid inappropriate use. For more in-depth info please visit the FAQ on premiers website.