Most of that stuff is crap. Most are filled with sugar, which if you are trying to lose weight its not going to happen. My suggestion is to drink water and if you want make yours from fruit and leave out the sugar (remember most juices are also loaded with sugar.)
As an occassional after work out treat (again try for more all natural) but as a steady diet no way. The number of obese people in a gym 3-5 times a week never ceases to amaze me.