A couple years ago I bought a Marmot Never Summer 0° sleeping bag, and I would have sworn it was the finest bag on the planet. It was certainly the best thing I’ve ever used: warm, well-constructed, with a roomy toe box and smart features like an inside pocket for a flashlight up by your head. This bag was ridiculously awesome–I even bought the model with a waterproof treatment on the outside. Best of all was the price: this was a <$300 sleeping bag, which I felt was completely fair given the level of technology we’re talking about.
And then I bought another sleeping bag, for Tracy. The one complaint I had about my Never Summer was that its warmth-giving bulk unavoidably made it about the size of a watermelon when I packed it. This time out I decided to upgrade to a higher fill grade just to see what all the fuss was about. Tracy thus got a Marmot Helium 15° (yes, a slightly higher-temperature, less-extreme grade of bag), which has 850+ rated fill. The difference in warmth between the two bags is exactly what you’d expect given their slightly different ratings. (Tracy isn’t likely to camp in sub-20 temperatures, which is what the “comfort rating” on a zero degree bag actually translates to).
But the real magic show happened when we packed them up. The Helium 15 is less than half the volume of the Never Summer. This gets to be especially important when you’re talking about traveling via airline to places like Yellowstone; cut the bulk of the bag and you can save yourself a $50 bulk bag overcharge in BOTH directions. The Helium 15 also costs right at $300 online right now, and its big brother, the Lithium 0, is $439 online.
The bottom line is that all this fancy down fill business really does translate into something. On a typical Yellowstone trip, packing an 850+ down bag versus a 600 fill “regular” down bag is very likely to save you up to $100 per person, since the bulky sleeping bag is the single likeliest thing to put you into the oversize bag category. (Couple this with Thermarest’s amazing NeoAir pad series and you can almost get away with a carry on bag).