Pie charts are everywhere and the casual observer might not notice they are generally a poor way of expressing data. Ask anyone who works with data visualization regularly and they will have an opinion, often a negative one, on the pie chart. As you delve into the theory of data visualization, you will most likely experience many of the five stages of the Kübler-Ross model of death. Hopefully, you'll come out in the last stage, acceptance, rather than depression. If you are looking for specific reasons pie charts don't work well, here are a few fantastic resources (here, here, here and here). Although the title implies extreme measures, pie charts (and their similarly delicious cousin, the donut chart) do have some value use in data visualization, although very limited (see chart below).
Stage 1: Denial
No pie charts aren't that bad. Here is a really good one I found here. They work really well in showing parts to whole relationships.
Stage 2: Anger
If pie charts don't do well to display large groups or really close percentages, then why use them at all? Why are there so many labels, markers, colors and legends needed?
Stage 3: Bargining
Ok, I'll use a pie chart only if I have 3 or less groups and they are really obvious. It is ok to use them only when talking about mutual fund constructions. How about I use a donut chart instead?
Stage 4: Depression
If pie charts suck so bad, why does anyone use them?
Stage 5: Acceptance
Ok, I totally understand the shortcomings of pie charts and will only use them when appropriate. I understand that this is very rare, but something I can use in my data visualization toolbox.