This week, Slate is running an article by me (!) titled, "How African are you?". It's about DNA testing for genealogy, and discusses some of the newer techniques and reasons for skepticism about how powerful they are in recovering genealogical details.
For those especially interested in proportions of ancestry in African Americans, I can direct you to the the Center for African American Genealogical Research, with links to many resources for beginning and practiced genealogists.
The real bottom line for any kind of genetic testing, is "let the buyer beware". Have a good idea what you expect the results will be, including the specific questions you want to answer, before you make any decision to send DNA away. These kinds of tests can answer certain questions, but if you are on a fishing expedition, you have a strong chance of being misled by the results.