Occassional fasts work well for me.
I have used 3 to 7 day fasts to kick-off a reduced calorie diet program several times over the last few years. Sometimes I have used a juice fast and recently I did the "Master Cleanse" or "Lemonade" diet.
It seems to have several benefits for me. As noted above, once you get past the first 2 days the hunger pangs do deminish - although for me they never totally go away. I definitely get the energy boost tauted by many, and it makes me examine my normal eating and drinking habits carefully and assess what I can, or need, to give up for a while. It also makes me realize how pervasive food advertising and the availability of food (mostly fast food) is. I never realized how bombarded we are with it until I stopped eating for a while.
My main reason for beginning a low calorie cycle with a fast is that I find it easier to say "no" to everything and then add back in the good foods I plan to rely on for the next couple of months, rather than continuing to eat nearly normally, but saying "no" to a few of the "bad" foods.
For the record I have pretty good eating habits, don't eat much fast or processed food, but I am a true "foodie" and like to eat well. I also don't really think that there are any "bad" foods, just some foods should be reserved for special occassions. In general, portion control is mostly the issue for me and I have a tendency to drink a little more good beer and wine then I should given my size and gender (smallish female), if I am not paying attention.
I definitely lose a few pounds quickly, but about half of them come back as soon as I resume more normal eating habits. But the good feelings of energy and vitality stick around. I do recommend fasting, but contrary to popular advertising, I don't think that it is some miracle csure, just another tool in the tool box.