As to the claims, they're dubious at best, as you can read here: http://www.diet-blog.com/archives/2006/03/09/special_k_diet_cereal_for_weight_loss.phpThat doesn't stop Kellogg's, of course, they enthusiastically promote their diet and even have an entire website devoted to it. See it here: http://www.specialk.com/challenge
Of course you'll lose weight if you eat fewer calories, so if you substitute a bowl of Special K, for a Cheeseburger and fries, you will lose weight in the short term.
The problem is, of course, that you won't keep that weight off. As soon as you return to eating the cheeseburger and fries instead of the cereal, you will regain the weight.
Real long-term weight loss comes from making real changes to your diet that you'll stick with for the rest of your life, not replacing meals with a bowl of cereal for a few weeks.