An outward bound school I worked at had probably a thousand sleeping bags, mostly slumberjack and including a few hundred Wiggy's. They were all 20 degree synthetics. There seemed to be no difference in loft or warmth between brands and after several months of use they all were colder by a noticeable amount. In the CA Sierra Nevada, summer temps were sometimes down to freezing and the more used bags were not warm enough. Even sleeping in long underwear and fleece, the students with the older bags were cold.