Oh dear, this is just a way for the cereal company to make money off you. It's sad. Of course it works- your bowl of cereal for lunch is about 250 cals, as is the breakfast one, and that means that you have about 500 cals plus for your dinner. You are basically just reducing your calorie intake in a simple way. You could do the Mars Bar diet (one for breakfast, half a one for lunch) or the lettuce diet (all the lettuce you want!) and you would still lose weight.
Like any diet the problems start when your body starts craving something else- cheese, bacon, eggs, bread, whatever. Restrictive diets never work. You are better off having cereal for breakfast, VARY what you have for lunch, and have loads of veg and protein for dinner.