These days never. At a high enough bitrate (like 320kbps) you'd need a good hi-fi to tell the difference. I can only tell the difference if I listen to one straight after the other. If you've got a high end sound system it's a different story of course. Even albums I have on CD I listen to on my computer. Also, without getting into a debate about the ethical side of music piracy, fact is I can't afford CDs these days. It's not a case of buy it or pirate it, it's a case of pirate it or don't hear it. If I didn't struggle to put petrol in my car and food on the table it would be a different story.
I know someday you'll have a beautiful life, I know you'll be the star in somebody else's sky...
But why can't it be mine?