i've got one. and for me it.... doesn't work!
my fender V has a dead spot on the 7th fret (D) and with or without it, makes no difference. still dead. i'm told where you place the ff is important as well, but i've tried pretty much everywhere i can clamp it on, still no results for me. maybe becos my fender V is extremely heavy, extra weight no point. but it doesn't bother me, cos i prefer using the high D on the D string. Roger Sadowsky swears by it though... and that probably makes me the a ff-idiot, heh