www.thenorthface.com
There are probably three aspects to a down sleeping bag, they being:
down
sleeping
bag
Errm. The more you pay, the better down you will get e.g. goose rather
than duck, higher percentage of down to feathers. This will reduce both
the weight and packed size of the bag.
e.g. if you look at Rab's 3 season bags, they use 85% duck down in
Atlas range, 90% goose down in Ladakh and 96% Polish white
goose down in Summit and Elite. The top bags are about 30% lighter (but
40% more expensive.
BTW, a good synthetic 3 season bag is probably half the price of a down
bag. It is still not as good value as these bags only last typically 3 years
good use, rather than 10 for a down bag.
Then you want to look at construction, e.g. do you actually fit (without
lots of air inside which'll circulate and reduce insulation - bags with
interior elastic help a great deal). Do you suffer from cramped feet (or other
cold spots - some bags have more down around chest area for instance)?
Does it fit snuggly around neck baffle + hood?
Typically bags use box or v (baffle?) wall construction, ie. interior dividers
are used to connect inner and outer fabrics, rather than being sewn straight
through (which'd give cold spots along the seams).
Another factor for sleeping comfort is inner and outer materials.
Finally, your bag will last a lot longer if you look after it. You must not store a
down bag in its stuff sack. You should wash it as little as possible - there
is a trade off between not washing and dirty reducing the loft, however, so
I always try to use a liner when weight is not an issue. You can get the thing
professionally cleaned when dirt becomes a problem (in my case every
3-5 years).
HTH.