Yep, I have one!! I think it is fab. Love it. People at my yard make fun of me, but it has saved me falling off many of times!
Basically I have it, as my mare is spooky and just darts back and drops her shoulder, she is quite jerky to ride and sharp in sense that one minute your on her and like a cartoon, I am in the air whilst has has darted off underneath me sropping her shoulder. So i just don't ride without it, she also spooks at speed and thinks nothing of stopping dead if she she's something, no matter what gait we are (OK, walk trot, manageable.. lol)
So this has helped so much. I can forget about what she might spook at in the school, as it just happens so quickly it's a real bugger to sit to.
It's like a handle/strap thing that you hold with your reins and if you feel you are about to become unseated you pull it up and your body forces you back down (I think?? - anyone else got a better explanation?)