The answer to the question posed in the title to this post depends almost completely on your own understanding of the results of a DNA test and your expectations. In talking to people recently who have taken a DNA test, I find their experiences and their reactions to the results of the test are heavily affected by the degree of their involvement in genealogical research and their prior knowledge of their family.
At one end of the spectrum are those who have done almost no genealogical research and know virtually nothing about their ancestors or their immediate family and at the other end are those who are seeking to answer a specific genealogical puzzle. Since some of those who take a DNA test have no online family tree, they can do little more than look at the general report and wonder about the conclusions. Those who were already doing genealogical research and used the DNA test to address a specific problem benefitted from the test. Those who took a DNA test out of curiosity are not inclined to pursue it any further. Obtaining some positive results from the test also depended on how many of their immediate family members had also taken a DNA test and shared the results or how many of those family members shared a common online family tree program.
However, I have seen extremely positive results for people who have done their "paper" genealogy and have specific research issues to resolve. The results range from finding a birth mother to clarifying a previously unknown relationship involving a child born out-of-wedlock. In one case, a family had to revise their entire assumed history due to the combination of both extensive research and DNA tests on family members.
There is a commonly held belief that taking a DNA test can be a prime motivator for doing family history or genealogical research. Although I have talked to a number of people who have taken DNA tests, I have not seen any increase in their interest in doing genealogical research.
To summarize, here are some of the factors I believe to me most important in obtaining positive results from a DNA test other than to satisfy a curiosity about your "family origins."
1. Extensive research preparation in formulating a specific question that can be answered by a DNA test.
2. Identifying specific relatives whose DNA will address the question being presented.
3. Making the results of the DNA known to potential relatives through posting on an online family tree.
4. Adequately evaluating and sharing the results to allow input from relatives.
5. Accepting the results as they are determined.In case of finding a completely unknown relative, the DNA results were published to an online family tree program and the potential relative contacted the originator of the website.