ok yes it is illegal, however i have no idea why it is... in england they are perfectly legal, the police see them as (quote from documentry)"a device that give the driver information", so yeh i dont really get y they are illegal, as by the time they beep and alert u if ur speeding it is most likely too late to slow down anyways.
i also saw a nice piece of equipment which jams cops radar guns, it gives the cop an error message on the gun, meaning they cant read ur speed LOL cant remember wat it is, but comes in handy...