I honestly don't know anything about Juice Plus and certainly have no affiliation with the company, but have performed research trials on other similar supplements and figured I'd give this thread a look.
Just looking through what has been said here, in theory, this can work. There are a number of products that have antioxidants in them and can do things like reduce oxidative stress and alter biomarkers affiliated with immunity and DNA damage.
I checked out the Juice Plus website and clicked on the research links, and the research is published in good journals from good universities. I don't have time to read through the articles, but the fact that there are that many articles which have gone through the peer review process strongly suggests there is something to it.
A few things to consider... Without thoroughly reading through all the articles (don't have time to do that), was JP just compared to placebo, or was it compared to actual fruit / vegetable consumption or general antioxidants? In other words, is JP actually better than other products out there, or is it just another antioxidant pill? My guess is it would be superior in antioxidant capacity to a few glasses of juice, simply because it is so concentrated. However, I am sure most of the fiber is removed to make JP, so it isn't the exact equivalent of eating all that fruit - it really depends what you are after.
The other thing to consider is the controversy about antioxidants and exercise. Are antioxidants beneficial for athletes? Or do they potentially prevent adaptations to endurance training? There is evidence for both - the issue is a complex one. So, as effective as JP may be at what science has demonstrated, does it ultimately improve athletic performance (if that is what your after) and if so, in what populations? In quickly browsing through the website, I did not see any research specific to athletes.
From what I can see, it appears to be a quality product. I can't speak to the actual value or how it compares to other similar products. 100% unbiased opinion.