There are a ton of articles out there explaining the disadvantages of pie charts, which is why they rarely turn up in our work. My good friend, Dan, is fond of saying, "You have three pie charts to use in your lifetime, so choose them wisely," however this article by Bruce Gabrielle makes some decent counterpoints worth considering (except the second point, pie charts are terrible at trends.) So if you've got a pie chart, consider if something else wouldn't be a better solution. Or you could just embrace it.

The slides for those interested: