Amazing Grass is not one of the most robust formulas on the market. It consists mostly of grasses, which are good, but should not dominate a green drink mixture because they don’t offer as much nutrition as some other botanicals. Get a complete review here with detailed ingredients and comparisons with other products. 
A good green drink is made from freeze-dried plants and vegetables (and often fruit or fruit juices) and is actually a food product — not a supplement, like a vitamin or mineral supplement. Green drinks are basically food in concentrated, powdered form. The best ones use organic ingredients and combine a variety of food sources. This article gives you a good first-look at the booming green drink market and helps you make sense of the products and ingredients in them.