I've always relied on a "set" of sleeping bags over the years and I think they have served me very well.  I've never gone to the woods without a standard poncho liner to accompany any of these.  
The first is a standard US Army ECW Sleep system, outer bag, inner bag, Gore-Tex bivvy sack, and 2" thick Thermarest mattress.  I wouldn't want to pack it for any length of time but I do enjoy sleeping in it.  Somewhere along the way I purchased a seperate but matching extreme cold weather bag to substitute for the outer bag, it probably brings the temp rating down to -40.
The second is a marmot -20 bag set (inner/outer) with Gore-Tex shell and a thin Thermarest.  Definitely packable and good for most winter conditions that you might want to try. 
Third is a spring/summer/fall set, a modified poncho liner with zipper installed to allow it to be bagged, a kelty light weight zephr bivvy and a section of parachute cloth for wind and rain deflection.