Been shopping for a sleeping bag that my fiance can put in his car in case he breaks down in a blizzard (Minnesota style).

My question: Just because a brand name says that the sleeping bag is comfy at -20 degrees F, is synthetic insulation material REALLY capable of that? This isn't a Wiggy's bag, mind you, though I'd love to be able to afford it. So when I look at other bags, which claim to be very good but are only a fraction of the price of a Wiggy, it seems a little dubious. How much money should I expect to spend for quality?

Any brand names that you guys trust? I've been looking for sales and at outlets and will pounce as soon as I know what to get.

Thanks!