The rating posted on your bag seems to be fairly accurate. bear in mind that several factors influence you comfort within any bag - the R rating of your padding underneath, exposure to wind, the caloric content of your last meal, your level of exertion. I find it really helps to wear a dry base layer to bed and some sort of insulating head covering when it will be really cold. Of course, your bag should be dry with the insulation well fluffed up.
Superusers do not speak on behalf of REI and may have received one or more gifts or other benefits from the co-op.