Pie charts are never fine. They're always useless chartjunk which serve little to no purpose. They're used all the time by people who should know better.

Even a case of comparing A vs B is better represented by bars than pie slices.

Bars are horrid for working out "what portion of a whole" is represented. I don't care what iTunes does - it looks like shit.

Pies give you an at-a-glance indication of whether something is more or less than a half (obtuse/reflex angle) or a quarter (right angle). This seems to be the sum total of their utility.

Stacked bars may be ugly but at least you can use the segments to make more precise measurements.

That.. I had no idea that kind of chart existed. I think.. I think i need to sit down.