My experience has been that I must fit test it first. I am too big for most standard bags, and even worse for mummy style. My current bag is a slightly tapered rectangular oversize weighing a little over 4 lbs and rated to 0 degrees. I have yet to test it short of the home trials, so I am not sure if it will be up to snuff for warmth retention or not. It cost me $60 on sale, which is about all I intend to spend on a sleeping bag.

I prefer synthetic fill to down. I've had the miserable experience of using a down sleeping bag in the wet, and will not willingly do that again. Qualofil seems to be the best for weight to warmth that I've found, but these newer synthetics also sound pretty good, though I've not tried any of them yet.

As with all my previous bags, I find something that fits and the specs look right, the construction quality is there and I will take a chance on it. If it doesn't work out, I will go back and find something else. You really won't know until you get it in the field and put it to the test if it will do what you need, but with experience you will have a better idea what to expect.

There's a big difference between the bag you will use at base camp and the one you will pack into the hills. Throwing a 20 lb stuffed canvas sleeping bag atop a king size cot in a wall tent with a wood stove is a luxury. Backpacking items are almost always a compromise, but there's plenty of functional stuff out there that makes for decent enough survival gear if you happen to have it on hand at the time.