Yes, I have used one. Stopped the saddle from slipping ever so slightly but I did feel that it pulled on the back hair quite tightly. Maybe thing about cancelling your order as you can buy that for half the price from your local garden centre
Ditto Billy .... my friend had one on a flat-backed horse and she ended up on the floor more times than on the horse due to it pulling the hair . Why are you needing one , I assume your saddle is slipping , to one-side etc . There are other things to try ( if you havent already of course ) girth on different straps etc etc