I have one. The one with gag type rings and it's a solid mouthpiece. Used it to hunt on a horse that could get strong and put his tongue over the bit. The few times I used it, he retired due to lameness, he went well in it. Less head tossing and general annoyance at not being allowed to tank off! He hated single jointed bits. Didn't mind ones with a peanut but this one seemed to give his tongue the room it needed.
I have one too, used it on my 5yo for hacking, fun rides and cubbing last year- she has now graduated into a myler comfort snaffle, but we use the happy tongue on anything that looks at all uncomfortable in the mouth and it almost always seems to do the trick!