I have two adult students who both use this device regularly.
One, because she was having some real pain issues caused by an injury in her uppermost armpit area, and the shoulders-back really relieved the pain.
LO and behold, she was also able to better balance her 3/4 Percheron around turns and to the jumps, and they were both remarkably straighter.
Now, mind you, I told her that we were to use this as a “lightbulb moment” and not as a crutch to keep her from developing the proper habits so she could carry herself that way.
It really helped her to understand how to close at the hip slightly, rather than tip over in the shoulders, to follow her horse’s head and neck without losing balance.
She still uses it about once a week to help remind her to keep that one particular collarbone/shoulder/arm stretched BACK.
The other student was almost the other way around. We used the first rider’s successes and lightbulb moments to realize how much she(th second rider) was collapsing her upper body and lower back during transitions on the flat. The Shoulders-back thingy made a huge difference in the response of this rider’s horse. She doesn’t use it all that often, but it was a wonderful teaching tool for her.
As for actually wearing it at a horse show… well… to each his own.
Can’t be any worse than people who pad the back of thier huntcoats so that it APPEARS that the rider has their shoulders further back than they do, or any other asthetic means to a better first impression with the judge.
Will never replace proper riding or posture, but can be a good reminder.
KD