Personally, the legality of sharing / downloading copyrighted music has never really entered my mind when I do it. When I do it, I don't think "oh crap, this is illegal, I hope I don't get caught!" More often than not, I think, "should I be doing this on the moral level?"
Is file sharing moral? That's a question up for debate, and has a much different answer than whether or not it is legal. I think that it's up to the person to decide it for themselves, and I have always been against forcing an almost purly moral decision onto others.
Some might say that there are many moral issues that the law enforces. However, I don't think that copying music is immoral enough in enough people's view that it warrants a law.
Others believe that illegal things are always immoral. Personally, I believe that's just a load of bull crap