I have one and used it once. Hard to see and hear in the daytime. It also has to be at the correct angle to be effective. If you have a 360 degree radar detector, rear detection will be effected by you sitting right in front of it.
You will have a better reaction time looking down the rode a ways for police instead of looking down at the radar detector. Ride with the flow of traffic or someone else who is moving a little faster. Your deceleration time will be much faster than theirs if there is police present. Whatever you do, try not to speed within the city limits in the open. You'll get caught more often.
I know the Ninja just says more speed please!