I watched the documentary several weeks ago and found it pretty interesting. But you have to put it into perspective. If you are severally overweight and eat junk food all day, a juice diet will cause you to lose weight.
The documentary makes it clear that the juice fast is supposed to be temporary as the guy did it for 60 days than switched to a pseudo vegetarian diet (only a few servings of meat a week). By eating a diet rich in vegetables and low in meat and other processed foods, you it is conceivable that you will be far healthier than the average person. The reason being that you are cutting out the junk foods that are epidemic in our diets. It's the long term that is unknown.
But I see the short term merits of the diet. If you desperately need to lose weight, it might be worth a try to do for 30 days.
Started 9/5/11 at 212lb
Last weigh in 12/10/11 at 188lb