i have never used one but from what i am told they work well, but they are not fool proof, you can't continue speeding after it goes off you have to slow down...So when it detects the laser it will scramble the speed showing an "error" for the cop.
However this is only to buy you enough time to hit the brakes because the cop can continuously pull the trigger and eventually he will get your speed. The scrambler in your car wont be able to react as fast as he can pull the trigger. Just things i have heard never tested it. Maybe when i buy a Ferrari i will get one.