Devices that inform you of the location are legal at the moment as are those that inform you of active radar style speed 'traps'.
Any device that refelcts or prevents your speed being read are illigal and would result in your arrest. An officer using a handheld speed gun such as the common ProLaser3 will very quickly click that your speed will not read and stop you anyway as theres few reasons why it would not 'lock on'.
Prolaser 3 opperate as, sight car pull trigger, speed displayed. So not even 1 second. However you should have seen the bright yellow jacket and white hat before then!
Save your money, keep your vision up and react to what you see. If your speeding you should be using heightened awareness any ways.