Wow, thanks a lot for all the great replies!

The reason I asked is because I took a one-nighter and borrowed a friend's 30*F down bag that was left in its compression sack for who knows how long. It seemed to take quite a while of "fluffing" the bag to get the loft back, and despite this, I was still pretty cold that night (it got right to 30*F). My feet really got pretty cold, but I think that since the bag was pretty long, there was too much circulating air space in the foot box. A jacket stuffed down there solved that, plus kept the jacket nice and toasty for when I got up.

I did like the fact that the bag was about the size of a football in its sack, and weighed next to nothing. I am a little dubious about the temperature ratings -- I think I would have been cozy at about 40*F rather than the 30*F the bag was rated for.

I think I'll consider some other options. Thanks again