I (correctly) self diagnosed (later confirmed by gastroenterologist) my Celiac disease through 23andme. But that is the easiest 1:1 gene correlation out there. My friend found out he had the same Parkinsons gene as Sergey Brin, and there are potentially things you can do to prepare. Exome testing may or may not be useful, depending on how much you invest in analyzing the results. I think it was of limited value until I ran it through Promethease.