I wouldn't say they run big as much as they run loose - haha. I have slim feet (that sounds boastful, but I'll take as much as I can get these days) and thin ankles and they are loose on my foot. That was easily remedied by adding in more holes on the bands for the buckles.
I have tried two or three different sizes on a few different versions of these, and my usual 8 was best every time. They do go on sale occasionally - like at the end of the season....