Best Bag Ever
Compresses well, immensely comfortable, unbelievably lightweight (a large Arnold Palmer weighs the same), and far warmer than its temperature rating suggests. The 850 fill down gives it an advantage over its brother, the Marmot Arete.
It's closest competitor is the Mountain Hardwear Phantom 45 but the Atom has a higher fill power, and additional fill weight contained inside. The Pertex coating is nice, allowing the bag to easily shed the cold dew of an early morning or negate any condensation problems your tent might have.
Get it, you won't regret it.