Radar detectors are legal across most of the United States, except in Virginia and Washington D.C. If you’re caught in Virginia, it’s a small ticket and no points. They used to confiscate your detector, but the law has been relaxed and they no longer do that. They simply write down your RD’s serial number instead. You can read the VA law here.
Radar detectors are also illegal on military bases, commercials trucks over 10,000 pounds, or any vehicle over 18,000 pounds no matter where you drive in the US.