Some horses like them, some don't like that chewy thing in mouth. Personally I wouldn't buy one for my horses, because I know some horses that chewed it through to the metal in less than a year. I'm pretty sure my paint would do it even faster. With that being said some people on forum use them for years with success. I use nice gentle Mikmar lorgenzo (spell?), and both my mares are happy with it.
I'm also not positive if those bits are legal in show ring (have to look up the rules), at least I haven't seen anyone showing dressage in happy mouth around here.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."