Yeah they are just an adjustable fuel pressure regulator sold in a kit with everything you need in one box, and with instructions which is handy if like me it was the first time Id ever messed about with the fuel system!
Useful for serious mods, nitrous, head work, wild cams, throttle body conversions, forced induction etc. Because you get to a point where you cannot physically get any more fuel flow through the injectors by increasing the open time. The only way is to get bigger injectors or increase the fuel pressure, which you need the pbv for. I cant see how it can just give bolt on power gains though, that sounds like b****cks to me. People have had good results though.