I did the Zone diet severals years back. I lost a lot of weight that I kept off and felt great. I also learned a ton about glycemic inde x, as related to the types of fruit I ate, which permanently shifted me to eating mostly berries. I learned things about food and reading labels that still help me today. It was a good plan to be on. Now, I did eat more fat than recommended and still lost weight and felt better for it. Where the Zone falls down is in the types of food you consume. Frankenfoods were ok if they met the 40/30/30 ratio, and that's where I started to have problems. But if you apply paleo thinking to the ratios, it's still not a bad way to eat.
I used to be on Zone forums and had people there telling me I was being too strict, because I decided that my carbs should be just fruits and veggies. People thought whole grains and bread should be a part of the carb ratio. Maybe that's why I lost so much weight on it, because I was basically eating primal. If I add in what I know now about good fats versus vegetable oil garbage, it's still a valid way of eating for health. I have to thank the Zone for teaching me that a serving size of pasta is 1/4 cup and not a plateful, and that my body will still process the pasta like white sugar. It was the background of the Zone that helped me jump right on board with paleo because i learned so much about how the body processes carbs while on it.