Well... there are two ways of looking at this issue.
Suppose you've heard about a new album, lets say Metallica's St Anger. You really like (or used to) Metallica and you have heard that this album is a "return to roots" one. You can ...
1. Either go and buy it
2. Dload a few songs and try it first.
I, unfortunately did the first option and I consider my money wasted!!! I hated the entire album. If I had first dloaded a couple of songs and got the general idea that the album was crap, I wouldn't have bothered to buy it... And if I had liked those songs then I could buy the album. This, in my opinion is the right way to use file sharing on the net. And most people I know, agree.
Now, supposing you dload a couple of songs, like it and then dload more songs of the band but don't buy any of their stuff, its like stealing. After all, how does the band benefit from your hearing their songs. If this keeps on continuing, most rock and metal bands (i'm not talking about pop artistes) would go out of business and then no one would want to really work hard to make music.
So, I don't think file sharing should be made illegal as long as you show your appreciation to the band by buying their albums. But supposing you don't like what you dloaded there shouldn't be any compulsion on you to buy it.
The words of a banished king
"I swear revenge"
Filled with anger aflamed our hearts
Full of hate full of pride...
Oh, we screamed for revenge
- Blind Guardian (Nightfall)