My experience is that you cannot manage your weight through exercise alone: in fact, the more you exercise, the more hungry you become! Exercise is an important and beneficial part of any weight loss plan, but it is just one part.
However, many of the diets you see are very dangerous. They are often very low calorie, and/or they do not provide the nutrition you need. You become tired and moody, and eventually your body goes into "starvation mode" - then you binge on all kinds of food, and become fatter than you were before!
Some diets will make you very ill, for example, some "low fat" foods contain so much sugar that you risk developing diabetes. Other diets (like Atkins) encourage you to continue eating unhealthy foods like burgers.
There are some healthier diets like Macrobiotics, Food Combining and Food Optimising. They encourage you to eat a variety of natural foods including plenty of vegetables and grains. I can recommend these, because they promote a permanent step towards healthy living
