In general it's hard to know what a chunk of the pie chart means. You get a very simple glance that one chunk is bigger, and another chunk is much bigger. But looking at that chart: How big is Eclair & Older compared to Honeycomb?
That chart is ordered by release; you start reading it at "3 o'clock" on the right hand side, and work clockwise.
Luckily they've labled each chunk of the pie. Normally they'd just have a key alongside it. People would have to match a shade of green (or some other color) to a key to try to get the numbers.
Pie charts do have uses. But they're just hard to read and present information in a weird way.