Hi
I bought the Rab Ascent 700 last sept/oct 2010 because I thought that a down sleeping bag would be warmer than the other one I had. I slept, or tried to, in temperatures no less than -3c and was freezing cold in this Rab Ascent 700 Bag. I wore thermals AND had a thermal sleeping bag liner as wel as others clothes eventuallyl, and also a sleeping mat. My partner, who does not feel the cold so much, said he would try it the next night, as he thought I was being a bit over dramatic. Well he, who does not feel the cold so much was very cold as well, and had all the thermal clothes and liners!
I would not recommend this sleeping bag to anyone, and when I contacted the company, they said that it depended how fit I was, how many calories I had consumed that day etc, etc, blah blah..I then bought a Mountain designs sleeping bag(in New Zealand where we were at the time), with lower spec, and it works MUCH better..no thermals, liners, etc.
Make your own mind up, but Rab saying that it is suitable for extreme temps of -31 as far as I am concerned is a joke!