tedster - 4:42 am on Jun 26, 2012 (gmt 0)

If visitors repeated continue searching after leaving your page, is a clear indication that user experience was poor.

Even that metric isn't so cut and dried as I see it. First, there are two categories of people who return to Google: those who make another choice from the same search results and those who revise their query. Sometimes the metric would show that the search results were poor and sometimes that the query was poorly formulated. And sometimes that the search user is doing resarch and comparing several differnt results - all of which may have been just fine.