疯 is very colloquial. 你疯了吗 is one of the most common expressions of disbelief. It's not as offensive as calling someone 'insane' in English — more like 'are you nuts?'
usage
疯子 (fēngzi) = madman/crazy person, 发疯 (fā fēng) = to go crazy, 疯狂 (fēngkuáng) = frenzied/wild.
