It definitely works and can definitely be done healthy, but it's important to note that the dude in the film points out that you should get a doctor's advice before doing something this extreme.
That's the thing... it IS fairly extreme. You can do something more reasonable and get results and not suffer through a few weeks of feeling like shit.
I did a thread on this a year ago you might seek out. I was trying to find HARD SCIENCE about things, but for some reason it's hard to find reasonable open-minded, but ready to call bullshit info on food.
At the end of the day, for someone as big and sick as the people in the film. Anything that works for them is good and beyond reproach IMO. It's fucking HARD to lose that much weight, keep it off and change your tastes/habits. I can totally see how, for some people, doing something extreme and cold turkey could help with a "fresh start" and doing it right this time instead of with middling half-measure.
The stuff in the film about "detoxifying" your body is complete bullshit though.
Why would you throw away all that perfectly good fiber? Juicing makes no sense.
As I understand it, the gritty and rowdy shit you get from juicing (not buying juice from a store) has the fiber. Thats a big reason people propose it only works if you juice fresh.