I'm not all that persuaded that the information it provides is going to be correct. The way it seems to work, they don't look back into the distant past with movements of people 30,000 years ago or anything; rather, they look at the family trees they have on file and compare your data with the origins of people who share some genes. This means it goes back maybe 1,000 years at most, so people have "British" DNA or "Irish" DNA or "Western Europe" DNA, but they don't have Celtic or Hibernian or Anglo-Saxon or Viking DNA.
Now I don't mind getting the results regarding where my ancestors were a thousand years ago. I do think they could be more detailed than "Great Britain". Also, the fact that it's based on what ancestry.com's family trees say means it could be based on frighteningly-wrong data. I've seen some absolutely amazing connections made — people who have emigrated to America a full ten years after they died in Yorkshire, that kind of thing. Also, there are many cases (I'd guess the majority) of people simply wholesale-copying pieces of family tree from some other user of ancestry.com, meaning that mistakes are amplified (honestly, I've seen 20 or more separate copies of a single tree that I know to be wrong because it's based off a record with a transcription error).
Furthermore, people often seem to have a desire to come from a particular place, so if you think your ancestors are Irish, say, you might be trying to connect all of them to Ireland rather than just the one, major branch that did come from there. Compounding this, records in some areas are much more complete than in others, so if you're looking for an ancestor in one location but can only find one with the right name and a feasible birth date in another area, you might be tempted to suppose your ancestor moved to the former from the latter when it's actually a different person — the records for your actual ancestor are no longer extant. You'll then proceed to go back several generations in that location, just like everyone else who made the same mistake, then make a similar jump when those records run out and "the only person who could be the parent" is from somewhere else. Bear in mind that to modern Americans, 100 miles isn't a big distance and you can see how this can be problematical if people construct trees casually rather than cautiously.
Taking all this into account, then, I'm not confident I'll get much information back that's going to be more than of passing use. Why, then, am I doing it?
Ah, well one thing where the test really does deliver the goods is in its ability to spot relatives who have also taken the test. I may come across people with whom I share a great-great-grandparent not because our family trees match, but because our DNA matches. That's why I'm doing it.
My wife is also going to do the test, but she's reluctant because she's worried she might be done for those murders she committed in 1983.