I just received a Kimberwicke in the mail a couple of days ago. I've put it on Mia once and walked along beside her and watched the bit & her when asking for a stop with the reins (couldn't try turns, since she will go where I go regardless).
On the mildest setting, it is still definitely a curb. Mild, but a curb. I bought one with a solid mouthpiece. Interestingly, she acted a bit freaked out when I went to put it in her mouth. And she responded very quickly to very light pressure. Before I owned her, she was sold to a girl for about 4 months prior to being returned, which was when I bought her. I'm wondering if they decided to fix her brakes (or lack thereof) with a strong curb. That might explain why she was so sensitive about her ears when I got her, too! If Mia was actually an unbroke horse at that point, as the trainer I hired last year concluded, then slapping a strong curb on and pulling hard would NOT be a good way to teach a stop!
We spent about 20 minutes walking around with quite a few stops and backups. She seemed much more relaxed at the end, and I then took the bit out and put it back in a few times without her fussing. I plan to ride her in a few hours, but since I'll be all by my lonesome I'll use her normal snaffle for riding today. Maybe put the Kimberwicke in & out a couple of times and do some more ground walking with it.
The longer I'm around horses, the more I believe in taking lots of baby steps in training!
I also agree with the function of the Kimberwick being too much for an unbroke horse to learn in...that would be rediculous.
The one I've ordered has a half moon lozenge. I have an eggbutt snaffle that I love with the half moon lozenge so I thought I would try this Kimberwick. The Kimberwick I will use on my percheron, who is fine with being soft with a bit. She is very responsive and I am light with my hands. I would just like to replace her current single jointed d-ring snaffle with the double jointed type bit. I think she will be ok with the added poll/bar/chin pressure since she was driven. I will go slow and test her out well.