While it wasn’t really the original subject of this thread, the subject of temperature ratings of sleeping bags has come up, and it is an interesting one, and some very salient points have been brought up.
Many inside and outside factors, such as sleeping bag design, shell material, mattress, tent, humidity, individual metabolic rate and so on, effect (or affect, help me, I never seem to get that right) the warmth of a sleeping bag. Yet given the same quality and quantity of down there is a limit to how cool (that is, how good) the temperature rating can be. Someone suggested, and rightly so, that if an excellent quality company like Western Mountaineering is rating a bag at a certain rating, then it is highly unlikely that another brand with the same down fill power and weight of fill is going to be any better. That is to say, Western Mountaineering does a good job at designing and rating their bags and if you see another brand (given the same quality and quantity of fill) that has a lower rating it probably is advertising hype.

Having said all that, the temperature ratings are still relative and relative to all the factors mentioned above. The free loft of the bag is probably a better indication of heat retaining efficiency than the manufacturer’s temperature rating because it is affected by not only the quality and quantity of the down, but also the design of the bag. The experience of others (reviews and friends) is valuable, but your own individual experience in your own situation and climate is the only proof of the pudding.
Thanks to all that contributed to subject here.