They have rightfully earned their nickname....
Kill
Your
Butt.
They are a crutch for a softly sprung car. The way that they function is that for the first part of travel, they are stiff....then they soften up as the suspension compresses..
This is exactly backwards of a decent shock that is initially soft but firms up through the range of travel.
As Andy wrote, in a car with soft springs, they will firm up the ride but in a stiffer sprung car, they are terrible.
I had KYBs in my car with 1.0 torsion bars and 440/Hemi leaf springs. It rode rough but I lived with it. Later, I swapped in 1.15" torsion bars but switched to Bilstein RCD shocks and the car actually rode better.Bigger torsion bars and better shocks but it actually rode much better.