The Gmail team reports the following on their Priority Inbox feature:
“Looking at median time in conversation view, we noticed that typical Priority Inbox users spend 43% more time reading important mail compared to unimportant, and 15% less time reading email overall as compared to Gmail users who don’t use Priority Inbox.
If you haven’t given it a try yet, use ClearContext’s Mark Important feature in v5.2 to achieve similar results in Outlook.