The Sprengers don't feel any different than a standard stirrup when you first ride in them; I am almost never aware of them flexing or giving. What you are aware of after riding in them in awhile is that your feet don't get numb, and that you don't feel the need to take your feet out of your stirrups and rotate your ankles to take a break. Then you become aware of less joint pain and fatigue. If I switch to a conventional stirrup, I can tell the difference in about 10 minutes of riding.
Very, very rarely, if I'm trotting in two point cross country at a good forward trot, I will feel the little bit of give to them.
I wouldn't recommend them for everyone, because they sure are pricey, but they are a huge help to folks with injuries.
My next big purchase will be the ones hung at 90 degrees - I have a lot of trouble picking up my off stirrup these days - my right knee really dislikes the lateral motion required to do so. I refuse to sit down to catch my off stirrup after a life time of walking off standing in my irons, currently I use my stick to hold the stirrup out to where I can catch it without angering my right knee.
Getting old sucks!