Pie charts, IMO, are almost never the correct choice. Even in cases where there's a logical summation to 100%, absolute values (the size of the pie altogether) is often also relevant; and pies with many slices are hard to compare, when the angles for different slice shadings are at different orientations.
Finally, too many charts rely on colour rather than patterns or markings to distinguish choices. That usually makes them hard to read for colour-blind people, as light shades of green and yellow, or dark blue and purple, etc. end up being used.
People can quantify the difference between two lines of disparate length much more easily than they can quantify the angle difference of slices of a pie chart[1]. Pie charts are evil and should never be used. A full 86.1% of this comment is fact; the rest is opinion.
Personally, I dislike pie charts for that reason, plus they're hard to scan. The text describing what area means what is floating all over, or connected by a bunch of parallel lines, or (arguably better) mostly absent / in a legend.
Bar charts, especially horizontal, have skimmable labels. Pie charts do not.
For those of us who lack graphical intuition (I'm guilty), look to Stephen Few (who looks to Mr. Tufte) for best ways to visualize business (and scientific) data.
Finally, too many charts rely on colour rather than patterns or markings to distinguish choices. That usually makes them hard to read for colour-blind people, as light shades of green and yellow, or dark blue and purple, etc. end up being used.