I bought a Reinsman Flex-tree saddle and it really made my back hurt when I rode in it. I don't know why. It didn't make my horse sore. The only other problem I had with it is that "rolled" on my horse and didn't fit securely when mounting. I also had it literally "roll" with me when my horse spooked and I got dumped.
So I personally can't recommend them. But the gal that bought my Reinsman is a friend and loves it. So it works well for her.
I think they would work best on a horse with a good set of withers, because I know a couple other people with round-backed type QH's, and their Circle Y flex trees are always wanting to roll when they mount as well.
I went back to having a couple of good wooden tree saddles and am very happy with them. I like flex-trees in theory, but in actuality, it didn't work well for me.
Oh, on another note. I weigh about 210, and my Reinsman flex-tree was a 17" seat. I couldn't imagine them making them that large if they weren't meant to be used by a larger rider? At any rate, my horse did fine with it, I was the one with the sore back!