I don't think it's a question so much as Special K being bad for you, in my opinion it's not an unhealthy food. It's more that Special K is unlikely to fill you up for long (well, in my case it fills me up for about an hour then I'm ravenous), maybe due to it's low calorie and protein count. Protein is what fills you up, so the more grams of protein a food has the longer it will keep you full. If you are a huge fan of Special K, I like the Special K Low Carb Protein cereal, don't be fooled by it's name it is not geared towards low carb diets but it does have good amount of protein per serving.
I love cereal, my staple breakfast cereal is old fashioned oatmeal. Many people on this board swear by Kashi Go Lean because it has a whopping protein count for a cereal (10 maybe? I don't have a box nearby to reference). It's tasty too. I don't like it as a breakfast with milk because like Special K, it doesn't fill me up worth a damn. I do like it as a dry snack though, or with yoghurt, mmmmmm.
August ~ First month on South Beach ~ lose 4lbs