LOVE IT!
I've used the Gentle Leader collars on my 2 Great Danes for years now. They are such a wonderful training tool, especially for big dogs that like to pull. It does take a bit of work to get most dogs to accept wearing the collar without trying to rub or paw it off, though a few of my clients that have tried it with their dogs had no problems with their dogs accepting it.
I can walk both of my Great Danes with my pinky finger using these collars and a leash connector! My female has gotten so good on a leash (only when the Gentle Leader is on her) that I can take her running with me when I go biking...with just a 4 foot leash. She does NOT pull me off, no matter what! Squirrels, birds, people, other dogs, trash cans full of food...no problem!
The Gentle Leader has also helped to prevent my male from biting someone (he's a bit over-protective!)...because I had better control of his head than would be possible with any other type of collar.
The collar definitely does work on a dog's instincts...both of my dogs are calmer and more obedient when wearing it.
I definitely recommend this collar
The only downsides are the training most dogs need to accept wearing it, and it can cause a bit of irritation at the bridge of the nose where the nose strap rubs if your dog does still try to pull persistently. I think they tried to alleviate this problem recently by making the nose strap a bit wider to distribute the pressure more.
On the other hand, even if my big dogs do try to pull with this collar, they can't really get their weight and strength behind it, so even if there is tension on the leash, they aren't dragging me across town! Even my 5 year old daughter can walk the dogs without being dragged.