I have one! Borrowed from my bodyworker initially and it made such a huge difference to my young mare I bought one.
I use it between saddle cloth and saddle rather than directly on the back.
Love that it has a really good 'drape' and moulds immediately to her back shape unlike gel mesh types or the prolite pads. Plus it's definitely true what they say about not bottoming out.
I wanted something that would not affect the fit of her saddle but would offer something to equally spread weight across a larger area of her back & protect properly from any bumps etc...: she's 4, super sensitive & had the whole summer on box rest due to a field injury... plus we struggled with stomach sensitivity when introducing her back to turnout ..so has lost all muscle tone & strength, is fussy around her tummy and still not very coordinated due to age!
Whilst obviously we're not doing a lot, the very first time I used the pad was the first time she properly stepped through and swung her back again wirh relaxed trot ears. Could have cried!
It's not going to fix any saddle fit or underlying musculoskeletal issues, but it does what it says on the tin and I am 100% glad I found it.