I used the Kelty LIght Year XP 20 this fall on a Mt. Whitney summit trip. The bag is very light, and compresses nicely for a synthetic bag (although the included stuff sack is a wierd shape).
My only beef is with the foot vents. While they could be nice if you overheat, I seemed to keep pushing aside the baffle in my sleep, causing a lack of insulation for my footsies.
I did use this bag down to 17 degrees F and was warm with my insulating layers on. (except for the feet of course)
I did return the bag, but would still recommend it if you're not pushing the temperature rating to the limit.