This is the first time I'm working intensively during the fast (I worked through the weekend) and I've been incredibly productive. I also prepared really well: I was eating more than a pound of dark leafy green a day for the last month (in two high micronutrient smoothies for breakfast and lunch) and I used a sauna everyday for the week before the fast. I feel better today than I have in a long time, my energy is excellent.
That's not to say there aren't any side effects, I had some pretty bad lower back pain which made getting to sleep last night difficult, but I've come to expect it and it comes and goes throughout the day usually starting on the third day. I occasionally have some heart burn as well, but it hasn't been as bad this time. My first fast was the most difficult, but it was medically supervised at the True North Health Center in Santa Rosa, California. They supervise in-patient water-only fasts up to 40 days.