The DNA analysis itself, trustworthy. The interpretation in terms of ethnic origins, not so much. Pricey experiment -- get your ethnic origins tested by Ancestry.com, 23andMe, and FTDNA. Compare the results.
One of these days, I might do that.
The analysis of ethnicity is based on statistical analysis of the frequency of specific alleles, and how those alleles have been observed to be distributed throughout the population. The reliability of those analyses is based on how many members of specific ethnic groups have been used in the original determination of allele frequencies in those groups. Obviously, the more members of each group submit to analysis, the more reliable the results become.
I am curious as to how the other DNA analysis companies would classify my ethnic origins.
Also, Ancestry.com asks people to upload their results from 23andMe, presumably to compare and analyze differences so as to provide a more reliable service.