Mark Manley wrote a review of MontBell U.L. Super Spiral Hugger #5 Down Sleeping Bag: 40 Degree on February 21, 2011
Just took this bag out for the first time and settled into it in about 35 degree weather. Realizing this was a 40 degree bag, not a more appropriate 30 degree bag, I was still expecting some level of warmth. I might as well been in a Coleman fleece summer sleeping bag because I froze my rear off all night. In addition to being also in a TiGoat UL bivy, which should provide another 5 degrees or so of warmth, I was wearing UL wool long johns, REI hiking pants, moisture wicking s/s shirt, level 3 capaline l/s shirt, Montbell UL down jacket, AND a Columbia lightweight nylon rain shell -- still cold! I'm not a frail or overly cold individual, so I have to wonder what's up?
Otherwise, the style and elasticity of the super spiral is great. I like the way it expands and moves with you. I may try to upgrade to the 30 degree bag..