Are radar detectors illegal?

My brother is going on a trip and asked if he can borrow my radar detector or not. I told him i sold it along with my car since i dont need it anymore. It's a very old model and i dont think it's reliable anymore. If he intends to get one of his own, i suggest he'd better go read up on those radar detector reviews before he gets one. Is anyone using one here? I heard that they are illegal.