I used to use one on my Fell x and he went very sweetly in it. I know a few horses that go better in them than gags and of course you don't need two sets of reins with them - so very good for strong ponies and children.
I really love them, I think they give a much simpler message to the horse than a gag and don't encourage them to lean. Also a lot harder to misuse than a gag. Would usually be my first choice for a horse that needed more than a snaffle.
I have one now and rode in one as a 15 yo on a BSJA pony..
I agree underrated and my ponies and I always got on with it..
In recent years I've only ridden in a French link snaffle or bitted, and bit my youngsters in French link snaffles but I keep a kimblewick around in case of needing a bit "more" in future..