I didn't mean to sound like a fanboy, but I honestly can say I HAVE seen positive affects on my health when taking the stuff. I was really doubtful at first too; the first time I ever bought greens powder, it was because I went into my local GNC-esque health foods store to purchase my regular supply of whey protein isolate, but the clerk at the counter convinced me to try buying some greens as well. I bought it, tried it out, and by the end of the first week, I could see the stuff working. I was having more "regulated" scheduled bathroom visits, my skin cleared up, and about a week and a half later, when I went in for a physical, my doctor, after getting back the results of my urine analysis, said my pH level was lower than before. Obviously, all of those things could've happened due to a combination of factors, but aside from the greens powder, I didn't change ANYTHING, from my workout schedule to my diet, to what I use on my skin, for months. The powder was the only "outside" variable.
If we think about it, all supplements at one point or another, have been controversial in whether or not they work. For a while, public hysteria even recommended staying away from supplements altogether, just due to the fact that the FDA doesn't treat them the same as prescribed pharmaceuticals. Remember the Ephedra scare? While some supplements (and their marketing) do make you wonder whether they are truly effective or not, I don't think Greens are in that camp. In terms of their health effects, while I might not buy into the whole "10 probiotic strains" advertising or "detoxify and purify the body!" claims, they do fundamentally work - fiber is fiber, whether you get it in the form of a powder, pill, pulpy juices or just raw food. If your just looking for more fiber in your body, are they the most economical solution? By all means no. But then again, if you were looking for fiber alone, you could just buy any number of products at your local supermarket. I personally wouldn't mind any additional health benefits resulting from a product I regularly consume. I don't expect miracles, but do find the product helpful. And judging by that, my personal experiences with the stuff as well as the opinions of my doctor, I stand by the stuff.
I would also like to add, for the record, that its not just the supplement companies that say the stuff works. Studies have been done by various groups that verify both the benefits of the individual ingredients that make up most greens powders and the products as a whole, as well as their their effectiveness on health:
The following groups have done research and have lent scientific and medical credence to the claims:
The Mitsubishi-Tokyo Pharmaceutical Inc.
The National Institute of Health
The American Society for Nutritional Sciences
The Linus Pauling Institute of Micronutrient Research for Optimum Health @ Oregon State University
The Harbor-UCLA Medical Center @ Los Angeles, CA
The UCLA Medical Center (separate hospital from above)
The American Journal of Clinical Nutrition
The National Cancer Institute