A dog must be trained to accept one and walk well with it. When you first use one, you are essentially re-leashtraining your dog, except they fell pressure on the snout instead of the head. Expect a little fighting at first, just like you had when you trained your dog with a collar. It is not as simple as just slap the thing on and everything goes great.
That being said, I find they can be a very valuable tool for some dogs. My beagle uses one when we are out, she seems more focused and her nose is not on the ground all the time. The best way to use one, is to introduce it slowly, with lots of praise and treats.
I prefer the gentle leader to the halti, they just seem sturdier. They can be a little hard to fit at first, it usually takes two people, one to fit and one to hold the dog. You have to watch out and make sure that the nose strap does not rub the fur off, some dogs are very sensitive.
Some dogs train to it very quickly, and some will just keep on pulling. Not quite as hard as before, but still pulling. I've heard some people worry about damage to the head/neck, but I've never actually heard of it happening. All in all, not a miracle tool, but they can be a good starting tool for teaching good leash manners.