Use bars showing percentage. This way you can compare the values to a scale on the side and to each other. You can't see a difference easily on a pie chart without annotation. With enough values, you likely can't tell 5% from 10%, and difference between a fragment on the left from a fragment on the right.
This is even worse in 3d pie chart which doesn't even preserve proportions in areas/angles, so splitting in 4 parts, bottom/top will be larger than left/right.
Pie charts are not about the raw numbers, they are about quickly and roughly showing who ate how much of the pizza. In many pie charts, 5% and 10% looking similar is a benefit, not a problem (because in many cases 5/10% are effectively the same - minor but non-negligible portions). Bar charts are not a great option for this.
I think it's better to say that "for most people there is a better alternative" than to say that pie charts should never be used.
EDIT: If you are showing who ate which parts of a pizza (to be pedantic) I'm sure a pie chart may be the best case solution. My favourite use of pie charts is to compare the harmonic series and the series 1/x^2, but then again this latter example is not really a traditional pie chart.
