Hi Peter. I’ve done research on pie charts and have written about them on my blog. First, we need to separate pie charts from 3D pie charts here. With a small number of slices, and for data that has a meaningful part-to-whole relationship, they work just fine. There’s a bit more on this on my website and I also gave a talk on some of my research a few years ago (search for “Unloved” on my website to find it, I can only post two links here as a new user).
3D pie charts are a different matter. 3D charts generally introduce lots of distortions that aren’t related to the data at all, and produce large errors in reading data. I have a forthcoming short paper at VIS this year where I used the distortion created by 3D pie charts to investigate the perceptual mechanism used to read the charts. Figures 1 and 2 in the paper should give you a sense of the distortion, and this is using parallel projection – perspective projection is a lot worse.
As to why people like them, my personal theory is that we inherently like round, complete shapes better than the jagged bar charts that usually work much better.
So in short, I’d recommend dropping the 3D and just going with 2D pies – assuming you don’t have a lot of data points in each chart and the data fits the chart. Adding the values is a good idea, but it’s really a cop-out. What purpose are the charts serving if people are just reading the numbers?