The Zone diet is a concept. You don't have to buy your food from them at all. That is how it started out as just the diet plan (no food).

I guess now the object is to make money.

The premise is 30% of your daily calorie intake in the form of protein. 40% good fats limiting saturated fat, and 30% low glycemic carbs.

Many people get rather confused on how to choose their food, so I guess they now make if for you. These percentages are not cast in stone. You could eat 40% protein 30% fat and 30% low glycemic carbs, and still do well.

By limiting or eliminating starchy carbs and sugars you immediately reduce insulin secretion and hence reduce inflammation. Balancing each meal and snack with some quality protein and low glycemic carbs keeps blood sugar in check and lowers insulin loads. The fat keeps food in the stomach longer, provides Omega-3 essential fatty acids, and energy with the lower carb intake.

The book Enter the Zone explains it all... and gives examples of meals that work. Over the years this diet has shown effectiveness in patients with insulin resistance and type 2 diabetes. It is very similar to the Metabolic diet or Syndrome X diet, which is higher in fat a bit. Those came out after the Zone, and are similar.
http://www.power-surge.com/educate/holt_syndromex.htm
The comment on that link, that the Zone limits Omega-3's is not accurate. In fact the first book from Dr. Sears, Enter the Zone explains why Omega-3 fatty acids are necessary to prevent inflammation.
We now have many "smart foods" and I even saw DHA milk at SAM's club yesterday! Smart Balance has many that we use in our house too including the peanut butter.

It used to be the website was useful for details in following this diet.
http://en.wikipedia.org/wiki/Zone_diet

You can probably find the book "used" for very little cost on Amazon.

Several celebs over the years have publicly stated they used the Zone...notably Jennifer Annistin--she was an early advocate. Here are some others:
http://www.zonedietinfo.com/zone-celebrities.htm

I think this method is easy to follow once you get used to it.
I still use the Zone Perfect bars as snacks for example. They have less sugar than other nutrition bars, and a good amount of protein and they satisfy as snacks or meal replacement. I prefer the fruit ones --but most of them are chocolate based which my husband likes better. They are now manufactured by Abbott Nutritionals.

I also avoid potatoes, because they give me burning....and that makes the diet even easier. Barilla makes a protein pasta called Barilla Plus, which I use too. Used in moderation (not large servings) it works for me and my blood sugars don't go up with it. I also find black beans very filling and helpful at meals instead of potatoes. Don't forget eggs...they are high in protein, and a good source of choline, which we need for our brains.