Anyone heard of this? Is it any good?  

Are you referring to the apparatus that just protects the PV from blowout?

If so, Holley has been employing something to similar to this for years. I think PV blowouts have become less of a problem, so I guess you could say it was successful.  

BB,

Your question is too vague for an answer to be specific.

Nevertheless, let me wander into your question.
When compression gases pop-up into a Holley carburetor, the power valve gets bent over backwards which jams the shaft and the power valve works poorly if at all and runs real-rich.

If we block the PASSAGE to the powervalve from pop-up pressure, we stop power valve damage from occurring.

Your question focuses on 'Power Valve Shield'. I haven't seen this as "Brand", but if you are speaking of the mini-needle / seat valve that is pressed into the base of the Holley--they work well.

Personally, I have been using a small ball bearing on top of the base gasket (between the main body and the base)for years, and it works great and it's free.

Roc