It is used usually in a mocking tone to remind someone that their situation is worse than yours. Person A: We have a test tomorrow, and I haven't revised at all. Person B: (who has revised for the test): Sucks to be you! "That's too bad" is another similar phrase usually used in a mocking tone. With close friends these phrases could be teasing, but would be rude and inappropriate in a formal context

It means it is not good to be you. It's like saying I'm glad I'm not you or It's acknowledging that you (the person they are talking to) are having a bad time. It could also be said sarcastically if someone is moaning about their life, but it really isn't a hard life.

= you are not very lucky. Life is not good to you. (Other meanings could be possible, depending on the context...)

I agree with the others. It's not very sympathetic, and should be used for someone you don't like, or who has brought the situation on themself. "I cheated on my girlfriend, and she broke up with me." "It sucks to be you." Of course, you can use it as a joke also, with the right kind of friend.

it's horrible to be you