I recently spent the night out in this Montbell bag and it was toasty. Temps, however, were only in the mid-30's, so I'm unsure of it's performance at the 15 degree rating. I was also inside my Cloudburst Tarptent on top of a 3/4 length Thermarest. By the way, I lucked out and got my bag, the long size, for $170 at a storewide 50% off store closure sale.