I did 23andme back before they had to stop doing some of the medical stuff. They discovered a serious and potentially harmful mutation that I had. I showed my GP who then confirmed it with other tests. It's something that could kill me, and knowing about it allows me to avoid situations where it could.

The ancestry stuff is really really cool too. I'm 2.7% neanderthal! My whole family discovered aspects of our history that we didn't expect.