1. Do a search on site:about.com on one of the major engines and see what you think about those results. 2. Search keywords related to the principal subdomain topics and see how often about.com has a nice position. 3. Explore about.com with the Yahoo Site Explorer [siteexplorer.search.yahoo.com] to see more.
Those three actions should show you a lot about what they do over at about.com and how successful it has been for them to date.