When I say that people should go on a juice fast asap, it is for the terminally and critically ill, those that have the most compromised digestive system of all. I've been there, and I had no choice as any food, even the healthiest, was going through me undigested.
If you're not healthy, a vegetable juice fast is necessarily not gonna make you feel great at first, because it is a powerful cleansing method. But I believe everybody can benefit from it, if proper liver and colon cleansing is done at the same time to assist the elimination process. One common mistake on a fast is that people consume too little juices and calories.
You're right, enzymes are always a great thing to add.