Well, I find this all very interesting. I really wanted to try a Nurtural Bridle -- they are made close to where I live apparently and there's been a lot of hype about them.
I only know two people that use them -- one had no success with it, but that lady has limited success with anything because she's very agressive; the other lady bought it and had great success. The horse responded far better. That was because the bit caused him physical discomfort, probably due to teeth changes at the time. Also, this horse is still in a learning curve so he didn't have much to "unlearn" and the lady did a lot of prepatory ground work with him. I agree with the poster that said to put a such a dramatically different piece of equipment on a horse requires some training first. I also agree that the mechanics of the Nurtural bridles are completely different than the standard headstall. Thinking about it now, I can see how this could be confusing for a horse well trained in a bitted bridle.
And someone mentioned the plastic spacer under the jaw on the Nurtural bridle -- this is to keep the cheek pieces in line. Supposedly, without that, the cheek pieces can ride up or down on the face and then change the pressures yet again. I haven't checked it out closely enough to say, but that's what I've been told.
Edited to add:
Oh -- my point from the start of my post was: now I'm not so anxious to rush out and try one of these things. I think I will stick with "if it's not broke, don't fix it"