I tried them at the recommendation of a trainer I had a while ago and wasn't much impressed to be honest. Very stiff elastic and I don't feel like they're easy to use to develop a contact. I found they give a very false impression of what the horse is doing with it's mouth. What was your reason for wanting to use them? As I think they may be useful in certain situations for example with a young horse learning to accept the bit maybe.
Personally, I hate the feel of them but I have seen a few people use them to good effect to improve their feel, trying to reproduce the same effect back in normal reins. They might also have some use with a more novice rider on a more sensitive horse stepping up their contact work, although I'm not convinced about their suitability as a "training aid" for a young horse.