All diets work on the same basic principle: if you burn more calories than you eat, you'll lose weight. The Special K diet does exactly that. It states that if you replace 2 meals per day with a bowl of cereal, (or other Kellogg's branded products - protein bars) you'll lose 6 pounds in 2 weeks.
Depending on how many calories this actually gets rid of will determine if it works. If you're just starting out a diet and your meals are big and full of calories, this will definitely help you lose weight. If you replace eggs, bacon and biscuits which can run around 800 calories with a bowl of cereal that's around 200 calories, you're already be saving 600 calories.
If you replace another large meal with a bowl of cereal, you'll save another 600 calories. On paper this looks really good. You're cutting out 1,200 calories from your intake pretty easily. However, when you try and work through a
day in which 2 of your meals are bowls of cereal or protein bars, you might think otherwise.
A good diet should also have variety. Sure they make different cereal flavors but at the end of the day cereal will always be just cereal. If you don't mind a lack of variety, try this diet out.
If you follow their guidelines you should be able to lose a good amount of weight in 2 weeks. If you're already on a diet in which your meals are small and frequent, replacing one of your low calorie meals with a bowl of cereal won't help too much.