I'm not aware of very much in the genetic material 23andme processes that is indicative of a 100% chance of having some future disease. Here's a screenshot of my 23andme dashboard with some of the information removed: http://i.imgur.com/9ViAf.jpg . As you can see, everything here is based on how much more likely you may be to develop a disease than the average person. Within the page details the specific studies that have given them the information being used in the calculation are cited with links and additional technical information. You can also see that the highest risk factor they've calculated for me is 11.3% or 1.61x the average (in this case, it's Age-related Macular Degeneration).

Obviously, it's possible that a person could be given a more significant percentage of a serious condition which raises additional questions. If you are both A) objective enough to firmly grasp that a chance is simply a chance, not a sure outcome and B) not someone who worries a great deal about possible future situations, you might benefit from using 23andme. Otherwise, it may be wise not to purchase the program.

For me, I've had an interesting effect of using some of the increased negative risks to help me stop bad habits while at the same time, not using any of the decreased risks as a license to go ahead and live more recklessly in those areas. Hopefully this give you some additional perspective on whether something like this would be right for you.