Are you talking about the Circle Y Flex trees? If so, I have one (the Topeka model) and love it. Really can't say enough good things about it. A very comfortable saddle that fits the horse really well.
The Ralide brand flex tree seems the most highly regarded, structurally.
Circle Y Flex (original) had some problems (defects) that Flex II supposedly fixed.
Supposedly all Circle Y flex trees sold 2006 and later are Flex II even if just marked "Flex."
Now Circle Y has discontinued the Flex / Flex II trees altogether, I hear.
Tex Tan's Tex Flex seems to not be complained about like the Circle Y is. I'm not sure whether it's the Ralide flex tree or not. Maybe Kevin "SouthnerTrails" can find out for you.
Flex is generally not recommended for swaybacks, and not recommended for riders 200# and over because those conditions bend the tree too much in the middle. Stress the tree and also creates weird pressure points that can injure the horse's back. I've read several posts where people say it injured the horse's back.
A good thick 1" top quality felt pad seems to be a must-have for a flex tree. Like something that costs $100 - $150 or so. Provides adequate cushioning and also support so that the flexing saddle tree isn't moving too much and "scrubbing" the horse's back.
That's just a summary of info that I compiled when I considered buying a flex tree saddle. I bought a good old rawhide covered wood tree vintage saddle.
Seems the safest bet for long trail rides and heavy usage. Wood with a rawhide wrap can flex somewhat to dampen shock for hose and rider. It's sturdy if you need to pull a rider or a log or something. Those saddle trees are sturdy and seldom break or come apart even after decades of steady use. They tolerable temperature extremes better than plastics and synthetics.
Plastics generally are considered to have a 10-year lifespan before becoming embrittled. Ralide trees are polyethylene, a molded plastic, and many of them last 30 years or more. I've read 2 people say that their Ralide trees cracked in extreme cold, like western USA.
All that said, some people love their flex tree saddles and use them with no problems.
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! :shock:
Flex trees seem to be an attempt to make the cheaper-to-produce plastic trees function the way a pine & rawhide tree has for hundreds of years. I would prefer them over a non-flex plastic tree, but I certainly wouldn't pay a premium for one.
Some more info here - describes some of the different brands: