I was a skeptic too. I was surprised at how much energy I had and how clear my thoughts became once the caffeine and fast-carb headaches subsided. Yogis and other ascetics have appreciated the benefits of fasting for millennia. If you are looking for hard data, look at the mouse studies that show increased longevity from being slightly underfed.
There is a lot of crap in our food and life. I'm no purist, but I do think its good to 'reboot' the system on occasion.