To be honest, Koreans don't say "당신" or "그대" as you in English. It's used in the past like way back in Joseon Dinasty but thesedays we prefer say "너" or "그 쪽(refer to you)" instead.

I never use it but some of korean learners use it..

I don't think you ever really want to use 당신. I think it tends to be really rude