I don't like pie charts. I don't like them because they fail to convey information. They do that because people have a really hard time judging relative areas instead of lengths. Wikipedia mentions some of the reasons why pie charts are generally poor.
I'd go a little further and say that pie charts are really only useful when a small number of categories of data are far, far greater than others. Like this image from Wikipedia of the English-speaking peoples:
Yep, there are lots of Americans.
Once you get data that isn't widely different or you have lots of categories your pie chart would be better as either a bar chart, or as simply a data table. Here's a particularly bad pie chart from a blog about Microsoft Office. It depicts the number of features added in various releases.
Literally eveything is wrong with this pie chart. The data being presented is the number of features added per release. Releases occur chronologically. So an obvious choice would be a bar chart or a line chart for cumulative information with time going from left to right. Instead we have to follow the chart around clockwise (finding the right starting point) to follow time.
And since the releases didn't come out at equal intervals it would be really nice to compare the number of features added with the amount of time between releases.
The pie chart has no values on it at all. We don't get the actual number of features, or just the percentage added. So we are left staring at the chart trying to guess the relative sizes of the slices. And that's made extra hard by the chart being in 3D. For example, how do Word 2000 and Word 2003 compare?
But if you still must use pie charts, I beg you not to use 3D pie charts. Please, they are simply an abomination. Making them 3D just makes them even harder to interpret.