I'm always weary of quick, get thin diets that you eat a certain food or drink a certain drink (Cookie Diet, Slim Fast, etc.)
I haven't heard of the muffin diet but I think anything that prevents you from eating a balanced diet of lean protein, complex carbs, & healthy fats is NOT a good idea.
There are so many options that offer a balanced diet. Personally, I follow the eat-clean diet lifestyle where I eat only clean, unprocessed foods and I've never felt better! (Even though I've lost only 6lbs so far, I look and feel so much better!)
A quick fix is not what any of us need! Slow & steady
(Sorry if this turned into a rant, I just hate to see when people waste time & money on stuff like this, we should be doing this the healthy way!!)