If it is the one on TV where the "chiropractor" is spouting about how good it is, I personally wouldn't waste my money on it. All it is is a fancy metallized decal designed to separate you from your $19.95 It is supposed to "passively re-radiate" the signal..from UNDER the battery
You might be better off to look to see if your particular phone has the capability to add an external antenna to help reception.
I would think that the thingy you attach to the ear piece to "direct" the waves away from your head would be closer to actually working, because if it was made out of wire, it would act kind of like a Faraday cage to help block the radiated signal form around the earpiece. But how good it works is debateable to me.
My .02