juice fasts?

Anyone tried these out? Pros n cons please?

I never did it. It is not healthy and the weight will come right back.

I've thought about doing a juice fast before, but I really don't want to smurf my brains out, lol. I've never really looked up the benefits other than knowing that if I drank juice for a few days straight then I'd feel like crap (mentally and probably physically) if I ate junk right after. My neighbor went on a fruit diet for a week before starting a new diet and he couldn't stop smurffing.

Pro: You lose weight fast
Con: You'll gain all the weight back

Im going it now but just for 3 days im only doing it mostly clear out my body is fresh for going vegan.
I do feel low energy today and my head is starting to hurt I just caved in and made myself a cup of miso soup and that seemed to help with my headache.