I feel like I'm often moaning about pie charts and then having to explain why I hate them, so I thought I should post here so that I can simply refer people to here for an explanation.
But when I Googled the subject it turns out that everyone else hates them too.
So there's really not much more I can say on the subject except for supplying the best link I found, which is a 2007 document by Stephen Few. It does a really good job of explaining how bad they are. It's pretty readable, but for the lazy you can get most of what you need to know from just the illustrations and their explanation.
Also here is the best quote I found on the use of pie charts:
"Piecharts are the information visualization equivalent of a roofing hammer to the frontal lobe. They have no place in the world of grownups, and occupy the same semiotic space as short pants, a runny nose, and chocolate smeared on one’s face. They are as professional as a pair of assless chaps. Anyone who suggests their use should be instinctively slapped."