I've never done a juice fast, but I have done water fasts several times for upwards of 10 days. I recently did one for 4 days prior to starting dieting/exercising about a month ago. It's a great way to purge the body of toxins that have been built up over the years, and of course it's a great way to rapidly drop some pounds.
From what I know, it's perfectly safe. It just has to be done correctly, prior, during, and after.
Anyway, there's an out of print book that has a great chapter on fasting, and it covers all kinds of fasts as well as documenting the health benefits and dispelling the falsehoods. You can find the entire book "The Complete Handbook of Nutrition" by Gary and Steve Null on Barnes & Noble right now for $1.99, but if you're interested in just the fasting chapter send me a PM and I'll point you in the right direction.