I just ran by Quantcast, which is still on beta but providing some interesting stats that some bloggers might like to check.

If your blog is lucky to be profiled, you can see some fine statistical analysis that estimates some figures about your visitors, like, their age, gender, ethnicity, even education and income.

The statistics also estimate your US unique visitors a month, as well returning visitors (loyal visitors). In fact, I guess that all the statistics are based on US visitors only.

One of the interesting results I found about my blog (sabbah.biz) is that 80 percent of the visitors are females.
Sounds I'm sexy :)
Not really. (Click thumbnail to see results)

Looking closer, I found that 85 percent of them have children ages 6-17, which means that US housewives like my blog ;)

So, if your blog is targeted at one of the above indexes give it a try (and pray that your female visitors are unmarried :))