Juice fasting is one of those "your mileage may vary" kinda things. The more things that are wrong with you, the more side-effects and general sense of crappiness you will feel during the first few days. Luckily for me, since I eat pretty healthy already, I didn't experience many side-effects at all. I did feel a little tired with periodic light-headedness, but other than that, it went really well! No headaches or extreme fatigue.
I've heard a few accounts of people having to stay near a bathroom during their first couple days. According to the one lady from the Fat, Sick and Nearly Dead film, she had to poop for an hour. In someone else's blog they said, "You can't talk about juice fasting without talking about pooping. A lot of poop." It was funny, but in my case, not applicable. My digestive system already functions as it should, with at least 3 regular bowel movements a day. So after I excreted what food was left in my system, there wasn't a whole lot left to push out. I did have to pee a lot though.
I expected to feel pretty bad the first day, and maybe even the second, but my first day was fairly uneventful. I did feel hungry now and then, and certainly missed eating. Food is one of the great pleasures in life. It'll taste so good when I resume solid food again! But my will is strong and I was able to resist grabbing a bite of anything. I think I'll have no problems lasting a week.