There are some misconceptions here about how the basic principals of radar works. Understand that and you'll know how a detector can or can't help you.
Remi is correct, when a radar unit is ON it will send out signals that detectors can pick up. This may be a wide angle where the operator can't discern what's causing the feedback (which car is speeding) or a more narrow and pointed direction with a specific target (hand-held operation with little or no other traffic). Fact is, most officers rely on the unit to be on most of the time and only take note when the tone is going off (this isn't always supported by the courts - law enforcement is to identify a speeding vehicle prior to operating the equipment for verification, not the other way around). So you'll find a lot of cars equipped with radar just driving around with it on, knowing an amount of speeders have detectors and therefore they'll slow down when they get a radar alert. Some municipalities even go so far as installing radar transmitters in high traffic locations as a decoy to make you slow down.
Anyway, there's enough info on the internet to figure out if you should need a detector. Personally I hate them - for one they're not always reliable, secondly I don't believe anyone should be exceeding the speed limit by such an amount that they require one (even in a properly equipped car with loads of driving experience), and thirdly it makes for a hellish time when you do get a t-stop and the officer sees it. It's like Christmas for him.
