What's the Deal with Down?
The 3D structure of down clusters create "loft" that traps air. The greater the loft, the warmer the insulation. "Fill power" is a measurement of that loft. This is accomplished by measuring how many cubic inches an ounce of down displaces when allowed to expand to its fullest. If an ounce of down takes up 500 cubic inches of space, then it has a 500 fill power; 700 cubic inches equals 700 fill power; and so on. Geese and ducks are the major sources of down insulation. Goose down is loftier (higher fill power) than duck down. Goose down is the preferred choice for lightweight products. Eastern European goose down is known for having extremely large down clusters that create very high loft, sometimes as much as 800 fill.
So, will a -20 C° sleeping bag with 700 fill be warmer than a -20 C° bag with 500 fill? Well, not exactly. They're both rated to -20 C°, but the 700-fill bag will have used less down insulation to get there. So, with all other things being equal, that bag will be lighter than the one with 500 fill.
Compressibility: Compared to synthetic insulations, down is typically more compressible, meaning that it can be packed into a very small space. This is great if you need to stow your down jacket or sleeping bag in a backpack.
Longevity: Down is very resilient and will retain its loft and ability to insulate for a lifetime if you care for it properly. Of all the reasons to choose down, this is one of the strongest.
Loft Factor: Down is a poor insulator when compressed. Down requires air to create a thermal barrier that retains warmth.
Performance When Wet: Down provides zero insulation when wet. Treated Water Repellent Down absorbs 30% less water than untreated down, dries 60% faster and retains much of its loft even when exposed to moisture. As long as you take care to keep your gear dry the occasional rain shouldn’t be a problem. However, if you know that it's going to rain continuously for days, synthetic insulation that keeps you warm even if wet may be a safer choice.
Price: High-loft down is usually more expensive than synthetic insulation, though if you take into account down's longer life span, it actually makes more economic sense over the long term.
Other Things to Think about
Whether it's for a jacket, vest or sleeping bag, the shell fabric needs to be "downproof." Down feathers can poke through fabric unless it's tightly woven. Take a look at the shell to make sure the down isn't sticking through anywhere.
Also, the shell fabric should feature a DWR (durable water repellent) coating or in some other way protect the insulation from moisture.
Check out Joe's for all your DOWN Needs.