Gwen uses the GoLite Fierce Sleep System. This sleeping bag is stuffed with 775 fill power down (down is lighter than synthetic, though down will not keep you warm if it gets wet--so make sure the fabric is water resistant) and has a temperature rating of 40*F. In addition, there is a synthetic (Polargaurd 3D) filled top that zips completely on or off the bag that will bring the rating down to 20*F. Just the synthetic portion itself is rated to 60*F. This bag allows a versatility in how much weight you carry. During warmer times, parts of the bag can be shipped home.
Gwen is 5'6", 120 pounds and fits comfortably into the small bag (reported to fit up to 5'6"). These bags seem to be longer than some others, and there is still a little extra room at the end of the bag. The bag weighs 2lb 3oz, and the top adds another 12 oz. Normally, this is a $350 bag, but we found it onsale from Sierra Trading Post for $112 plus shipping.
This is a well made bag with draft tube, neck collar and full zip. The hood is easy to cinch down, and the zippers don't snag on the fabric.