Disclaimer: I am an on-off subscriber to Ancestry but have not taken their DNA test. I have however done yDNA, mtDNA and autosomal DNA tests with ftDNA. I do not claim any expertise on DNA and what follows is purely the personal views of an amateur in this area.
Autosomal DNA
In my opinion this is a pseudo-science and there are more than a few charlatans offering tests in this area. The reason why I say it is a pseudo-science is although there is some science behind the subject there are too many unknowns and assumptions for the claims that are made for the ethnic predictions made. As some posters have observed you get different predictions from different suppliers when using the same raw data. Indeed I have suggested to several people who have taken autosomal tests and been unhappy with the results to use the Gedmatch.com site where they can get several different analysis done on their data with some quite wide variations in the results. I think this is particularly important for anyone who took the Ancestry test as there was some criticism of it when it launched particularly that it was overestimating Irish and Scandinavian ancestry, predictable perhaps given their main market i.e. the US.
To give an example of the differences you can get ftDNA give my ethnic results as 33% Scandinavian, 67% Western and Central European. Using Gedmatch and their Eurogenes K36 Admixture analysis (one of many available) gives the following make-up Basque 1.96%, Central_Euro 5.62%, East_Balkan 1.98%, East_Central_Euro 6.22%, Eastern_Euro 4.87%, Fennoscandian 6.80%, French 10.36%, Iberian 13.77%, Italian 8.24%, North_Atlantic 18.46%, North_Sea 19.09%, Volga-Ural 0.10%, West_Caucasian 1.00%, West_Med 1.54%. Are these two analysis consistent, yes in my opinion they are but as you can see they are widely different. This is why I see autosomal DNA testing for ethnic analysis as a bit of fun and not to be taken at all seriously.
Where Autosomal testing is worthwhile in my view is for those trying to find family members. However this is limited in that autosomal testing for this purpose is only good for about five generations back and is seriously limited by the miniscule number of people testing. Hoping to get far on this requires a bit of luck or a long wait. That said I did make contact with a 4th cousin once removed in the States and we have since confirmed the findings by the good old fashioned paper trail. (There are several closer matches but they can take longer to confirm than people think).
So to the question should I take an Autosomal test my answer would be yes if it was to find relatives and you had low expectations of results. It would also be yes if you were curious on the ethnic mix and were prepared to treat it as a bit of fun, but no if you were going to take it as gospel and therefore potentially be disappointed with the results; as more than a few on this board have been.
I would also advise anyone taking an Autosomal test for the ethnic analysis to also use a comparison site such as Gedmatch to see how much the analyses can vary.
Cost
Several people have commented on the cost question and yes all the tests are costly for what they provide. The way I have explained this in the past to people who have queried this is to say that it is a developing science and you are not just paying for the test but you are contributing to the research to make the results better over time. Some people are prepared to do this and some are not, fair enough. However, there are some companies that have jumped on the DNA testing bandwagon and the fees they charge are way out of line with the more main stream companies. These are the charlatans I referred to previously, they are not in it for the science but just to make a quick buck, but as always buyer beware.
Why test
The reason why I initially took a yDNA test is my direct paternal line comes to a halt in the middle of rural Northumberland in the 1740’s. My hope in taking the test is that eventually someone from a parallel line will take a test and help with the way forward. That was three years ago and as yet nothing but as the longest I have had to wait for a confirmed genealogical fact is thirty years I think I can wait a while longer. The mtDNA and autosomal tests I took were just to complete the set, I did not have any particular expectations for them they were just curiosities. However they have confirmed factors that my yDNA revealed
