Anyone can lose a couple pounds on a 2 day juice fast-basically because you are not eating for two days, and because having nothing but juice gets things "moving" in your bowels. The Hollywood 2 day juice fast type thing are basically having you down to about 300-500 calories a day. (There are a few different brands of them which very calorically.)
Most of the weight you are losing is water weight, and the cleaning out of the bowels.
It isn't good for your blood sugar levels though-ESPECIALLY if you are the type who can get dizzy, etc. when you haven't eaten. I would pass out. (borderline diabetic)
Also, you are getting no protein, fiber, calcium, and many other things for the days that you are on the fast. There is no protein, so if you are exercising to increase your muscle and reduce body fat-a juice fast will actually work against what you are trying to accomplish through exercise.
Lastly, there isn't anything special about the juice detox/juice fast blends. If you went on 400 calories of Kool-Aid, or 400 calories if Pepsi a day-it would pretty much do the exact same thing...and you don't have to pay $15-$20 a bottle for it.
It is still basically not eating, and a few hundred calories of sugar into your system a day, given in small increments.