Both are true. 당신 is very honorific. But if you use it to your lover it can be very touching, like "당신을 사랑합니다!". And if you want to start a fight with somebody you can use it as well. Because it can sound like you make a fun with the person. There is a nice free lesson about this word where you can see how exactly to use it. I am not sure can I post some links here, so if you can't find it feel free to contact me and I will give you the link.