I also love SSvsSS - it's great for illustrating that skinny can be just as unhealthy as obese, although in different ways.
I think it's also good at highlighting the many different reasons that people are over/under weight. Some of the obese people know nothing about nutrition at all, but others know exactly what they should be eating and often do, but just love food so much they eat way too much of it! There are emotional eaters, those with no willpower, those in denial, those in ignorance...etc., etc,. I like that the show gives us all something to relate to with the different people they show. It's not just highlighting how bad it is to be overweight...it shows the full spectrum.