I've used the powdered stuff in the past. Superfood. Greenfood. Supergreen. I can't remember what it's called, but the same concept: a days worth of fruits and veggies in a big scoop of powder. I mixed it in my V8 everymorning for a year or more a while back.
I definitely felt good during that time, but that is far from a controlled experiment - I was treating my body well in many ways during that period.
We've recently been given the powder to our picky-eating daughter. She thinks it "chocolate milk". hehe. You should see what she calls a "cookie". We're so mean.....