Make sure you have the right pad!
I do not own this bag, but have a BA Yampa 40 deg down bag. I have repeatedly used it in low 30s temps (ice on the tent, frost on the ground, expected temps in the town 2000' below is 33 deg) and been warm with at merino wool LS top and hiking pants (I am a warm sleeper). I DO have the INSULATED Air Core pad from BA that is rated down to 15 deg. The NON insulated pad is only rated to 35 deg. Because the BA system does not insulate the bottom of the bag, it depends on the pad to do that. Basically you can get a 0 deg bag and it wont work well below 35 if the bottom of the system is not insulated. I am not sure this is the other reviewers problem (insufficient pad with the bag for the temps) but if you want this to go to 15 deg, MAKE sure you get the INSULATED Air Core pad.