I got one in 2008, and although I was a bit annoyed at first, when I'd had chance to calm down, I considered myself to be quite lucky, because I was in the wrong but fortunately the copper was on his own and had no speed measuring equipment in the car.
As said above, they last for a year and it applies to both the car and the driver.
As far as i know, if you get 2 in the space of a year, they can sieze and possibly crush the car.