luke salzmann replied:

macka i am assuming that you are a big bloke so you probably a warm sleeper the answer is not that easy you could probably go down to about -3deg which means that you will be comfortable at about 0,+1 after that they turn into mummy bags which aren't that big and they don't open out like a sheet to add a couple of more degrees you will have to add a sleeping bag liner we are also talking about down here so you are probably looking at close to $300 to $500

Will replied:

Macka,
Get a "Roman" Model Big BrotherHood (Which means big brother with Hood) you can also get it without a hood. it is huge but also big to carry and it is rated at -8.
I have never ever been cold using it and it can be zipped to another one (same model) to create a double doona or a double bag.
Will