Blu:Are you in Canada or Europe?
While the covers dont work very well at all, against Laser, they do work very well against stationary photo traps.Those tend to look down from an off angle and the covers can make the license unreadable on the resulting picture.
They are illegal in some jurisdictions for this reason, but they are also difficult to distuingish, for an untrained policeman eyeballing them from his cruiser, so the risk of getting a ticket just for having one is very very low (and in any case that would be a LOT lesser ticket than these tend to protect against).
In the US this technology has been used almost exclusively at traffic lights to capture PPL running a red light, which I hope you are not planning on.
If in Europe or Canada stationary devices on poverp[asses, lamposts etc, look for speeders.If I lived in a jurisdiction that used them for this purpose I would probably get them.
Fred...
PS: Additonal info: They dont work by blocking radar just by making it more difficult to take a picture of your plates.Some are also made to make lasing you more difficult, as police are trained to lase onto your front (or rear) license plate, if this second effect is your intent, you probably will be disapointed by their lack of performance in this area.