Don't start raging when you find that the device you are purchasing isn't up to par. Lets put it this way, I have the "clone" to the Valentine detector and this bad boy was 350USD 2 years ago. Just don't get pissed off at the merchandise because it isn't 100% effective. Chances are you will notice the cop at the same time your detector reads it as a priority signal!
Here is a review from car and driver.
Obviously it will be a little effective but don't TAKE YOUR eyes off the road ahead of you!