I just don't believe in it. Years ago, I used a loaned PowerBrake for a studio recording and my Marshall Super Lead died withtin 2 hours. Also, I didn't like the sound.
Wasn't the introduction of the first master volume amps the first 'Power Brake' of sorts? I never liked these too (except for my Fender Blues Junior).
The sustain/saturation just isn't the same. I'd rather use an amp modeler or plug-in instead of castrating a perfectly good amp. Though there's nothing that can beat a good amp in its unleashed state....