Back when I worked as a clinical dietitian at a local hospital, I worked
with a family whose daughter (about 2 y.o.) had many grand mal
seizures.  To prevent her from hurting herself if she had a seizure while
in bed in the hospital, she would have to wear a football helmet.  All
types of meds were tried, but none seemed to help as much as was
desired.  We then tried the ketogenic diet along with the meds and did
see improvement.  However, this was an isolated case.  And like Deena
said, it was not an easy diet to follow - very little in the way of
carbohydrates, with lots of MCT (medium-chain triglycerides) oil to boost
the calories.  Not a very palatable diet.