I have the original Herm Sprengers, and I probably couldn't ride without them. I have an ankle held together with screws, wire and bone grafts, one total knee replacement and one knee that needs to be replaced.
I often wonder if I had had the Sprengers sooner, if I could have limited some of the knee damage.
My Sprengers absolutely feel no different from the saddle than a standard fillis iron; the difference is I can ride for much longer and that I can walk away from the mounting block after I dismount.
I definitely concur with the advice to try as many different brands as you can before making a decision. I don't know if it's absolutely necessary to spend the bucks on the Sprengers.