I have had both the Comfort and the Ultimate models (the original ones). I first got the Ultimate (the flex kind), used them for a few months, and felt really unstable with them, especially over fences. I didn't like the give and started doing some weird stuff with my legs to try to stabilize myself. Keep in mind I also had never ridden in any flex stirrups before.
I ended up selling them and getting the Comfort model (same but no flex) and love them. I wouldn't use anything else now. I used to get rubs on my calves from the leathers and don't any more. I also tried using my old plain fillis irons again and found my foot going numb and I felt very stiff.
There are plenty of other threads about these stirrups if you do a search.
I've spent most of my life riding horses. The rest I've just wasted.