Advantages – This style of eating has no obvious food limitations and allows flexibility in food choices. There are no restrictions especially those which are commonly restricted on other styles such as gluten, meat or dairy. You simply have to stay within the macronutrient balance within each meal.
Furthermore in 2010 a study from the American Journal of Clinical Nutrition found that nutrient deficiencies were less likely on this style of eating than they were on other popular weight loss diets. (http://ajcn.nutrition.org/content/92/2/304.full)
It’s also worth noting that in a comparison trial between The Zone Diet and other common diets a 2005 study published in the Journal of the American Medical Association compared adherence rates and the effectiveness of 4 popular diets (Atkins, Zone, Weight Watchers, and Ornish) for weight loss and cardiac risk factor reduction. Each of these diets modestly reduced body weight and several cardiac risk factors after 1 year. However, fewer Zone and Weight Watchers dieters dropped out (about 35 percent) compared to Atkins and Ornish dieters (about 50 percent). Thus if compliance really is the most important factor for long term success with weight loss, the Zone Diet may be an easier way to stay compliant in comparison to other methods. (https://www.ncbi.nlm.nih.gov/pubmed/15632335)
Disadvantages – The style of eating suggests a certain proportion of carbs, protein and fats at each meal. This obviously can become difficult in terms of planning meals and often requires significant planning in advance.
The diet had a theory for being beyond weight loss, and it was suggested to have an effect on the role of the ratio of insulin and glucagon in your body. The suggestion was that this led a cascade of effects that reduced chronic disease risk, improved longevity etc. If you require a little more detail, this is from Dr Barry Sears himself.
'When insulin levels are neither too high nor too low, and glucagon levels are not too high, then specific anti-inflammatory chemicals (types of eicosanoids) are released, which have similar effects to aspirin, but without downsides such as gastric bleeding.'
Sears claims that a 30:40 ratio of protein to carbohydrates triggers this effect, and this is called 'The Zone.' Sears claims that these natural anti-inflammatories are heart and health friendly. Whilst many purported benefits have been long associated with a loss of body weight in heavier individuals, these claims directly as a result of eating a 40/30/30 macronutrient ratio are not fully supported in the scientific literature. https://www.ncbi.nlm.nih.gov/pubmed/12569110
To Sum It Up
So the Zone diet may be easier to stick to in comparison with other diets, and Dr Sears also promotes exercise, healthy fats and cautions around refined grains and sugars if not restricting them completely. Futhermore he suggests a strong intake of fruits and vegetables. However beyond the weight loss effects which will be attributable to being in a caloric deficit, there are no large scale research studies to support any other claims around the diet.