Personally, I don't like music, so I don't have much of an opinion on mp3's, but from time to time I do download software from file sharing program.
I am a big fan of the
"Free for personal user, but not for corporation" and
"try before you buy" mentality.
Sometime, when I learn of an interesting program, I will go on a filesharing program and download it. If I like the program and I can afford it, I will buy it. If the program suck, I will promptly delete it and use my money for something else. If the program is good, but I could never afford it, then I might keep it for personal use.
I see nothing wrong in this, as I still buy things I like and I save money by not buying bad products. I believe the same thing could apply to mp3's.