Since my father has early-onset Parkinson's, I have given this question some thought and decided against it.
Being cleared by the genotype does not give you a license to neglect your health. So the generally positive result is for the most part useless, if not slightly detrimental to a healthy lifestyle.
Conversly, your options for prevention are limited were you to find the increased risk of Parkinson's, Alzheimer's, or a similar neurodegenerative disorder. The little you can do to protect yourself against genetically-inherited illness, you should be doing even if your genotype is clear. At the same time, knowing that you are at risk for something unpleasant down the line could be psychologically difficult for the rest of your life, and downright dangerous if you are already predisposed to hypochondria or depression.
There are exceptions to this line of reasoning of course: if you have a history of diabetes in your family for example--diabetes being a disease that responds well to behavioral modification. Or perhaps in the case of an Ashkenazi couple getting tested for Tay-Sachs before conceiving. Outside of these specific exceptions, I don't see any benefit to the test.
"It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge. " --Enrico Fermi
If you had knowledge of early-onset Parkinson's or Alzheimer's, you could plan for it by making sure to, say, spend much more time with your family than putting in lots of hours at the office with the expectation that you would be promoted later. You would also be able to manage some of them better with early knowledge and expectation of what is coming, and be able to volunteer for drug trials. (It may also affect your planning... (read more)
I've heard several people here mention getting genotyped by 23andme (I assume there are other companies as well). Does genotyping have significant practical benefits or is it mostly for curiosity's sake? If there are significant benefits, what are they?