I have never come across a situation that calls for a pie chart. The human mind thinks linearly: we can compare lengths of line segments but when it comes to angles most of us can't judge them well.
The donut chart is a pie chart with a hole punched in the middle. Alas, the missing middle contains the angles that help us size up the slices. The donut chart is a useless chart made worse. Never ever use a donut chart.
Each publication gravitates to certain "pet" charts: the Economist happens to like donut charts. Hopefully their editors will read this and stop using them. Here is a recent example:
We might as well point out three additional crimes: firstly, having one donut as a mirror image of the other denies us any chance of comparing like-colored slices properly; secondly, the lines linking labels to slices positively make us dizzy; finally, the least important detail, i.e. the total population size, stares us in the eye.
Reference: "The Americano Dream", The Economist, July 14, 2005