It's rare that a $500 product is a steal, but the features and performance NEMO packed into the Sonic make this bag one of the best values of the year.
Start with top-of-the-line, 850-fill duck down—same warmth as goose down, just cheaper—that ensures toasty comfort down to the 0°F temp rating. A hybrid baffle design trims cost relative to traditional vertical baffles: Since the vertical baffle chambers are only 26 inches instead of the entire length of the bag, they don't require pricey, internal mesh-wall construction to prevent down migration within the baffle. Also, the vertical alignment keeps the insulation over your core (it can't migrate from side to side) where you need it most.The horizontal baffles through the legs have stretch stitching for extra mobility—enough for one tester to change inside the fully zipped bag on a freezer-burn morning in Wyoming's Wind River Range.
In warmer temps, unzip the two longs gills on top of the bag to vent core heat; it's a big improvement on the cold toes we get by opening the footbox on conventional bags. Even with all this stuff, the Sonic is still at the lightweight end of its class. Only downside: The hood's bungee cord hangs on your face if you pull it all the way tight.