I just learned about this review article that was published earlier this summer in the European Journal of Clinical Nutrition:
A. Paoli, A. Rubini, J. S. Volek and K. A. Grimaldi. Beyond weight loss: a review of the therapeutic uses of very-low-carbohydrate (ketogenic) diets. European Journal of Clinical Nutrition 67:789–796, 2013; doi:10.1038/ejcn.2013.116
This is a fantastic article summarizing the current knowledge about ketogenic diets. Read it!
Update: The article lists the following as diseases with strong evidence that ketogenic diets can be beneficial: obesity, cardiovascular disease, Type II diabetes, epilepsy. The article lists the following as diseases with emerging evidence that ketogenic diets can be beneficial: acne, cancer, polycystic ovary syndrome, neurological diseases (e.g. Alzheimer's).