I have done this loads of times, usually in a club - and I really do mean many, many times. Taken off our jeans and underpants and then swapped underwear. I always wear white briefs, usually Calvins. Whenever I see a guy also in white briefs I always try to get his underpants and go home in them. It is such a turn on right from getting your first foot through the legging to pulling them up completely. And a continued turn on as you leave the club knowing you are wearing another guy's underpants - and that he is wearing yours. You become very conscious you are wearing underwear (normally you never feel what you are wearing) and your arousal can often last a very long time.