Radar detector laws in NJ?

i just bought a Cobra® XRS 9445 14-Band™ Digital Radar/Laser Detector and was wondering about the NJ law about them. They say its legal to have in a passenger car, but the guy i bought it from at radio shack told me its illegal to use...so are using radar detectors illegal in NJ? What is the fine for being caught with one? How do they catch me? If they can catch by seeing it, cant I just hide it somewhere in my car?