Without having gone through the thread to see if someone has addressed this, I'll just say they're kind of crappy. Human history and ethnography going back in time usually doesn't match up with geographic boundaries. There are a few ethnic groups, like Finns or Ashkenazi Jews, that can be traced, but most of the time it's not like you can say "you have this gene which matches up with this ethnic group", it's not that simple to interpret. It turns out people have babies with people from other ethnicities, all throughout history, so mixing is the rule not the exception. So you might wind up with a certain percentage chance that you have ancestors from a vague region that encompasses many modern-day nations, which may or may not be satisfying. If you have done some geneological research and you want to confirm that great-great grandpa Joe said he was English but actually was Irish, or there's a family story that some ancestor was half Cherokee, there might be a chance to confirm it but you' likely have to contact some 2nd and 3rd cousins to submit their samples. And at that point you're probably interested enough to consult a professional geneticist. Same if you have health questions, you'd definitely want to talk to a genetics counselor along with your physician.
A while back 60 minutes featured an African American woman who wanted to know about where her slave ancestors came from. It's a deeply personal question for meaning, and she likely wanted to travel to the part of Africa where here ancestors lived. So the show paid for her test, and it came back with some area of western Africa, and she was moved and emotional (understandably). But then the reporter says "we also sent it to another testing company", and they opened the results, and they did not match up! Completely different geographic area and ethnic group. So the woman was like "Wait, WTF?" It's just not (always) as straightforward to trace back as these companies make it seem.
