I've done a juice fast. The amount you lose enormously varies depending on how much and what kinds of juice you consume. If you drink only beet, cuke, and greens juice, you may lose. If it's mostly fruit juices, that's more caloric. I think the main idea is that the nutrients are more immediately absorbed by your cells when so much less digestion is required.
Juice fasts really apply to freshly-made juice, not storebought juice. I have the small model of the Jack LaLanne juicer and it works great. You might also consult juicemaster.com or Jack Lalanne's site for juice recipes.
Some people suggest consuming very little and diluting it with water. There's some point to that, so it doesn't slam your body so immediately, but it's not so tasty either. I like to compromise by making the juice and then blending it with ice.
Mini-Goal by 1/4/14
Main Goal by 2/1/14
One per 5 lbs!