Below is a lecture from Christopher Gardner Ph.D. at Stanford. He describes a study that compares the weight loss and general blood work (cholesterol, glucose, etc) of four major diets - Ornish, The Zone, Atkins and LEARN.
The study was designed to see how people did when they read a diet book, then followed it for one year on their own. From this study, Atkins beat the other three in both weight loss over time and overall health based on blood work.
Gardner is a 25 year vegetarian.
YouTube - The Battle of the Diets: Is Anyone Winning (At Losing?)