I actually own a neck stretcher/ neck relaxer, although I dont use it because I dont have a need for it any more.
I agree with the people who dont like training aides, and agree also that collection comes from behind.
Sometimes the horse can have all the push in the world from the hind end, but if they lock their neck (typical of the horse that tends to get nervous) they will never come on the bit. If you have exhausted your options, or are getting frustrated with the 20 minutes of flexing the neck every time the horse gets nervous, then the neck relaxer is a nice tool. It encourages the horses neck into a position where the muscles have to somewhat relax, hence the name.
But make sure like someone else said that you are keeping contact with your reins. Other wise you are simply letting the neck relaxer hold the horse up, which is going to incorrectly teach the horse to hang.
The idea is to encourage correct muscling so that when you do ride with out it, the horse is strong enough to hold himself there as well.
The other nice thing about the neck relaxer is that although it feeds through the bit, it essentially uses pressure at the pole to encourage a correct head set.
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."