I haven't but my wife and sister-inlaw have on American from LAX to IAD last summer. My wife had discount ticket and sister-inlaw was flying free; she only paid taxes. They had free companion ticket deal. The redeys flight was full and they wanted to sit together. Without saying anything, gate agent gave them the boarding pass and they were for business class!
Personally, I was upgraded once but I had Y class ticket. BTW I fly about 18K miles a year...