It depends on:
- which type of "greens drink" you're talking about
- how often you are drinking them
- why you are drinking them?
- when are you drinking them?
The best scenario is to choose a good product, use it occasionally to boost your vitamin and mineral intake, drink it away from meals, as a snack, and heaven forbid! don't mix with any sugar, and only a minimal amount of juice.
There are many powdered greens products out there: such Greens+, Berry Greens, Triple Greens and many others. These are typically freeze-dried, powdered green vegetables and may include "superfoods" such as algae, spirulina, wheatgrass (baby wheat stems). The powder can be added to juice or water and drinking it supposedly gives you an energy and nutrient boost. Some juice companies, such as Odwalla, or Happy Planet's bottled products or Booster Juice or Jugo Juice stores will add freeze-dried, powdered greens to their drinks.
It's always better to eat fresh, whole foods such as broccoli or kale instead of consuming their powdered form, because then you're getting all of the benefits of the food, especially the fibre. However, food science is now pretty sophisticated, and a freeze-dried vegetable can contain almost as many nutrients as the fresh version. Some companies, such as New Chapter, use organic fresh fruits and vegetables and ferment them before freeze-drying to ensure that all the nutrients in the food are fully available before drying them.
What's the fermentation all about? Foods like spinach and kale contain enzymes which prevent their vitamins and minerals from being digested and absorbed unless they are broken down by cooking/fermenting, or eaten in combination with other foods such as dairy. Of course, once you cook a vegetable, you immediately lose some of its vitamins. Fermentation offers a great way to unlock and preserve a food's nutrients.
What about these superfoods? I personally don't like eating any of these green "foods" which humans wouldn't normally eat in nature. Spirulina, wheatgrass and blue-green algae may be mean, green and packed with vitamins, but I shudder with nausea when I drink them. I prefer to stick with greens products, such as Berry Green, which don't include these unusual foods. On the other hand, the company that makes the Greens+ line offers flavours and variations for everyone, as long as you don't mind the taste of spirulina.
While I really appreciate the convenience of juice stores like Jugo Juice, I find that like most companies, they'll sell you cheap ingredients to make maximum profits. Sometimes their fruits are not very ripe, but when they're frozen, and tucked away in a hidden freezer, it's hard for the customer to know. Until you taste them that is. So that's why they mask the flavour of unripe fruits with ice cream or "sorbet" as they like to call it. If you go to a juice store, try to get fresh juices, made from vegetables instead of fruit, and make sure they do not add ice cream or any other sugars.
Eating greens powder could be similar to eating the equivalent of up to 2 servings of organic green vegetables and superfoods. A small amount contains a large amount of vitamins and minerals. And it's a pretty convenient way to get these nutrients: at work, on the go, or first thing on the morning. But remember: a greens drink is not as good as eating those fresh veggies, so if you are able to prepare delicious leafy greens, you should do so, and not depend on powdered substitutes. I'm envious of people who have the time to cook nutritious meals filled with fresh, organic vegetables, because I'm not one of them. As long as you don't depend on your greens drink as the main way to get "vegetables" into your body, then you're doing your body good.
Again, if you want to drink your greens, be sure to choose a greens product that has a wide variety of leafy green and cruciferous vegetables, don't mix it with more than 200 ml of juice, and don't rely on it as a crutch to preparing and eating lots of fresh vegetables each day.