If you mainly camp during spring, summer and fall, and at elevations where the temperatures are not likely to drop below about 20 degrees Fahrenheit, then a three-season bag will work best. Three-season bags are less expensive and lighter in weight. But if you enjoy camping in the winter or at very high elevations, you'll need a winter sleeping bag.
The down-filled GoLite Adrenaline 0 (*Est. $400) weighs just 2 lbs. 9 oz., and earns the 2008 Editor's Choice Award at Backpacker Magazine. It was tested in rugged conditions including "bone-chilling nights at 14,500 feet in Nepal," and editors say "even the cold-sleepers on staff reported blissful warmth." The half-length zipper saves weight, as does putting water resistance at the head and foot, where you're most apt to touch a wet tent. The main drawback is that the short zipper limits ventilation.
The REI Kilo Expedition -20 provides excellent ventilation – with two layers of zipper on the side – and is rated to minus 20 degrees F. Owners reviewing it at REI.com give this winter bag mostly very positive reviews, praising the water resistance, hood design and adjustable ventilation. About a third of the reviews say the zippers tend to snag – and it's a bit heavy at 3 lbs. 12 oz.
Most down sleeping bags use baffles to separate the down on the top of the bag from the down underneath, but Feathered Friends, another top-ranked manufacturer, takes a different approach. You can move the down from on top of you around to underneath, depending on where it's needed most. Feathered Friends sleeping bags are made in Seattle, so repairs can be made right at the factory, and users praise the customer service as excellent. The zero-degree Snow Bunting comes with your choice of shell fabric, either EPIC (*est. $475) or eVENT (*est. $525), and weighs 2 lbs. 13 oz. on average.
For car camping – where weight doesn't matter as much – owners like Cabela's Alaskan Guide Model Mummy 0 Degrees, which weighs 7 lbs. 5 oz. This hybrid winter sleeping bag uses 650-fill down for the top insulation, but synthetic Thermolite Extreme synthetic insulation underneath. It also comes in lower temperature ratings: Minus 20 Degrees and Minus 40 Degrees as well as in longer lengths.