I'm a very casual viewer and don't have a strong opinion one way or the other. But my impression is that they show a lot of marathon workouts and very little meal planning, cooking, and contestants restraining themselves from overeating. That would make for extremely dull television, but it's what makes for the vast majority of weight loss.
Also, when the gay kid from Utah spent the entire first week vomiting and then lost 40 pounds at the first weigh-in? I pretty much screamed at the TV "OF COURSE HE LOST A TON OF WEIGHT HE SPENT A WEEK PUKING!!!