I have. I am not sure they are all they are cracked up to be, but I am not sure that they are not.
They tend to make you ride with your toes hanging off the front of the pedals, which isn't a bad thing at all, but it gives you a false feeling of quicker acceleration. Well, the acceleration is faster, but I believe that is only because you are further forward on the pedals. When I adjusted them so that my feet were in the normal position, I discovered that my acceleration was about the same as it was pre-PowerKnobz.
Where they shine is in the transition from standing to seated.in fact, I never felt comfortable trying to transition until I got the PKs. It is beautiful and seamless.
One con I noticed is they give you plenty of "wiggle room" for your legs. For me, it meant I have to clamp down on the sides of thewheel more when hopping curbs or riding bumpy sections of the Greenway. Previously, I relied on my pads much more than I thought. The result is kind of a "sloppy" ride feeling, which I assume will go away over time. On the other hand, some may find it liberating to have some help accelerating without relying on pads.
I also noticed I tend to use my feet as a lever to control my speed, rather than having them be mere support characters. After a 25 mile ride, my feet didn't hurt as much as they do with regular pads, likely because I am using my foot muscles in a different way.