Most good down bags will be supplied with a much larger cotton bag for storage, so that the down is not stored compressed.

The best down bags will be half the weight of the best synthetic bags. The only downsides to down is you have to ensure that the down bag does not get damp or wet as this will lose the downs ability to loft properly and the ability to keep the sleeping bag clean. Sythentic bags can be regularly washed in a washing machine unlike down bags so a liner for a down bag is a good idea. A Pertex or Silk liner is recommended.

Down will also last considerable longer than any synthetic bag in keeping its lofting properties for much longer periods of time. Its probably good practice to keep any sleeping bag whether synthetic or down dry. Sleeping in a wet bag, whether down or synthetic is pretty horrible. If using a bivvi bag then it's probably not a good idea to use a down bag with a non MVP material. This also applies to the sythetic bag also to a slightly lesser extent.

A good down bag also has that comfortable cosy feeling that is very difficult to achieve with a synthetic bag simply becuase the way the down draps across the body.