Um...despite all the drama, I will comment on this, since my young one is currently in a kimberwick. The trainer actually started him in a snaffle (which is also what I used when bitting him for the first few weeks), then moved to a baucher (mullen mouth, not broken), then to the kimberwick. This is not a bit I would usually consider (especially since it is not dressage-legal), but he seems to go well in it for now...I would like to wean him back to a baucher, at least, if not a plain snaffle.
With the snaffle, he was very, very fidgety and strong: constantly chewing, fussing, got his tongue over it several times. He would run right through it, as if it weren't even there. You could almost see him "tuning out" so he could play with this bit like a dog with a chew toy. Anything with a broken mouthpiece showed the same results.
With the solid bar of the mullen mouth, he was better about the fussing, but it was determined he needed a bit more space for his big fat tongue
With the kimberwick, he has been the least fussy and seems to go happily. It is only used on the high slot (least curb action) with a loose chain, so it works similarly to the baucher, but the low port seems to give his tongue enough clearance to be comfortable.
Again, I was fairly surprised when this ended up being the best solution for him at this point in time. Just thought I'd share my story and the logic behind the choice.