Any replacement diets do not work long term, but can be helpful to jump start weight loss. I just started the slimfast shake for breakfast and lunch, but I know in a few days I will omit one, and have a meal. I'm using it to transition into a diet, and I guess because I like seeing that initial drop in the scale!
If you wanted to do special K for two weeks and then slowly transition over, that would be fine. If you just go off the diet though, and don't make a lifestyle change, you'll just gain the weight back.