I have done 23andMe and had my full genome sequenced. Unfortunately, we are still in the early stages of understanding our genome and there is often little of medical importance revealed.
The most significant of the "something really bad and unexpected" are the risk factors for Alzheimer's and Breast Cancer. I believe that there are certain markers for both those diseases which could reveal that you have a much higher lifetime risk (for some cases of breast cancer, >50%), which is obviously scary. Don't quote me on this, but I believe in both situations you are very likely to have had some family history in these diseases, so it should not be a total surprise. And if you have a family history of breast cancer, you should be getting Myriad's full BRCA test (see my other post). I would probably look into a clinical test for Alzheimers too if I thought I was at risk.
There are a few other diseases that also have big impact (>50% lifetime risk), but they are rarer, and I'm not sure if there are any where you would have had no family or personal history to clue you in on the possibility.
The vast majority of the diseases 23andMe reports on give you very little actionable data. For example, it might say I have a 2x risk of prostate cancer, from a background risk of 2% chance to a personalized 4% chance. I view that as pretty unhelpful. Furthermore, it's based on SNP-associations currently known. It's possible, I have another SNP that's associated with a .25x risk, and therefore I actually have only 1% chance overall, i.e. half the general population's risk.
23andMe can be useful for other reasons. For example, it will tell you if you are a recessive carrier of certain diseases, which is helpful if and when you decide to have children. It's also fun, has ancestry info, etc.
The most significant of the "something really bad and unexpected" are the risk factors for Alzheimer's and Breast Cancer. I believe that there are certain markers for both those diseases which could reveal that you have a much higher lifetime risk (for some cases of breast cancer, >50%), which is obviously scary. Don't quote me on this, but I believe in both situations you are very likely to have had some family history in these diseases, so it should not be a total surprise. And if you have a family history of breast cancer, you should be getting Myriad's full BRCA test (see my other post). I would probably look into a clinical test for Alzheimers too if I thought I was at risk.
There are a few other diseases that also have big impact (>50% lifetime risk), but they are rarer, and I'm not sure if there are any where you would have had no family or personal history to clue you in on the possibility.
The vast majority of the diseases 23andMe reports on give you very little actionable data. For example, it might say I have a 2x risk of prostate cancer, from a background risk of 2% chance to a personalized 4% chance. I view that as pretty unhelpful. Furthermore, it's based on SNP-associations currently known. It's possible, I have another SNP that's associated with a .25x risk, and therefore I actually have only 1% chance overall, i.e. half the general population's risk.
23andMe can be useful for other reasons. For example, it will tell you if you are a recessive carrier of certain diseases, which is helpful if and when you decide to have children. It's also fun, has ancestry info, etc.