I had to Google Banshee to remind myself exactly what it is; turns out it’s "a female spirit in Gaelic folklore whose appearance or wailing warns a family that one of them will soon die." Oooo-kay. Sign me up for that!
Now it’s true that excessive exposure to wailing female spirits – in particular Ann Coulter – can make you wish you were dead, you will be blissfully oblivious as you snuggle up in this zero degree bag from Mountain Hardwear. Stuffed with 800-fill down and covered with a welded, box-baffled, waterproof and highly breathable Conduit SL laminate shell, it packs down small and sheds everything short of a full-on deluge. No worries about condensation here.
The Banshee weighs in at just under three and a half pounds, with a pound and a half of that being fill weight. It has an ergonomic mummy cut, draft tube and collar and two small pockets You’ll pay for all this wonderfulness, but if you need a fast and light waterproof nookie bag at this temperature rating you’ll be happy when the fit hits the shan. In general the temperature rating seems about right with all the usual caveats: get a good pad, be hydrated, fed and not tired, and all the other things that are impossible in your average epic.