While juice fasts, or as they are sometimes called "feasts" are popular it's wise to consider your own health before you do it. I would also make sure it's okay with your doctor. If you notice, on most of the movies like "Sick, Fat & Nearly Dead" which is one of the movies that has made this very popular the participants go to the doctor to make sure they can try it, and then also to get regular checkups to ensure nothing is going wrong.
Juice can adversely affect some people with diabetes or other conditions if they are using too much sweet fruit. So be careful!
If you've not watched the film: Sick, Fat & Nearly Dead it is on netflix. I recommend it.