DNA testing companies advertise their products as interesting and exciting, but they don't tell you much, if anything, about the potential downsides.
Everyone I know, myself included, has discovered surprise relatives through DNA testing. Sometimes it's just a cousin. Other times, it's a half-sibling or a child or a parent. In some families, this may cause considerable upset. Other families embrace their newfound relatives without hesitation.
If you discuss DNA testing with your family in advance, their reaction may inform you of potential surprises in your results. You may decide to take the test anyway. You can obscure your identity on the testing site with a nondescript user name and a private family tree. If you do that, you may receive heartrending messages from people who are desperate to find their biological relatives. (Please be kind. No matter what happened, it wasn't their fault.)
Even if you obscure your identity or choose not to take the test at all, there is a chance that your surprise relatives may find you anyway. Some of your first or second cousins may have already taken the test, thereby exposing your shared DNA to family seekers.
DNA is inherently not private. We share it with all of our close relatives and even some of the distant ones. As recent criminal cases have demonstrated, you don't have to submit a DNA sample in order for people to deduce your identity.
There are also some concerns about testing vendors and shared databases. What exactly will they (or could they) do with your DNA? Some privacy advocates worry that taking a DNA test could affect your ability to purchase life insurance.
Many people take DNA tests to learn about their inherited health risks. Even if you did not test through 23andMe, you can obtain health information by uploading your raw data file from Ancestry or Family Tree DNA to Promethease.
Keep in mind that this is an emerging science. Your risk assessment can change, and very few diseases are strongly associated with a single genetic mutation. In most cases, there are other genes which mitigate your risk.
If you belong to an underrepresented population, the DNA test may not offer much useful information at first. When my step-father took the test two years ago, he had very few close DNA matches. Now he has dozens.
People seem to get very excited about these, but ethnicity estimates may not be genealogically useful. They may not even be correct. When AncestryDNA updated their ethnicity estimates last October, I went from 12% Italian to 0% Italian overnight. Is the updated estimate accurate? The data will probably improve as the databases become larger and more diverse.
DNA testing is interesting and exciting. Please discuss it with your family first, and be aware that assessments of your ancestry, health, and ethnicity may change as new information becomes available.