This paper, written by Stephen Few of Perceptual Edge, presents a range of arguments on why you should not use pie charts. His overall message is that bar graphs provide a much better means to present and compare the information in a graph form.
"Pie charts are not without their strengths. The primary strength of a pie chart is the fact that the message “part-to-whole relationship” is built right into it in an obvious way. Children learn fractions by looking at pies sliced in various ways and decoding the ratio (quarter, half, three quarters, etc.) of each slice. A bar graph doesn’t have this obvious purpose built into its design. Not as directly, anyway, but it can be built into bar graphs in a way that prompts people to think in terms of a whole and its parts. This can be accomplished in part by using a percentage scale. It is easy and natural to think in terms of various percentages in relation to the whole of 100%. Seeing a bar extend to 25% along a quantitative scale conveys a part-to-whole relationship only slightly less effectively than a pie chart with a quarter slice, especially if the bar graph’s title declares that it displays the parts of some total (for example, “Regional Breakdown of Total Revenue”). Despite the obvious nature of a pie charts message, bar graphs provide a much better means to compare the magnitudes of each part. Pie charts only make it easy to judge the magnitude of a slice when it is close to 0%, 25%, 50%, 75%, or 100%. Any percentages other than these are difficult to discern in a pie chart, but can be accurately discerned in a bar graph, thanks to the quantitative scale."