Some people say you shouldn't kill the goose that lays the golden egg. Well I say screw them. They obviously haven't frozen their ass off on top of some godforsaken mountain in Lesser Trashcanistan. I say kill the damn goose and make a sleeping bag out of it.
Feathered Friends is a Seattle-based company dedicated to doing just that. If it can be made with down, they make it. The Ptarmigan EX -25F is a shining example of fart sack technology, at just 3lb 11oz with 36.5oz of soft and lofty 850+ fill power down. It even comes in a women's (Plover) and big-boned (Peregrine) versions, the latter useful for when you want to get in with your other down gear on. Kind of like a human/goose version of a turducken. A gooman, if you will.
A nice detail is the lack of nasty velcro inside to stick to your undies. One snap closes the outer bag, another snap fixes the draft collar. There's one drawcord for the collar and another one for the hood. A nice fat draft tube runs behind the full-length zipper. Feathered Friends uses big, roomy baffles to allow their premium hand-picked down to fully loft, and they take extra care in how they stitch it all together. Our only (minor) gripe is that the bag isn't available with one of the water-resistant down options, although to be fair the shell material they use does pretty much the same thing.
The result is a bag that is true to its comfort rating and then some: do not hesitate to trust their temperature ratings. An added bonus is the choice of shell material they offer: it comes by default in Pertex Shield EX; I got mine in waterproof, breathable eVent, which will stand up to prolonged condensation and evaporation in multiweek outings. For faster and lighter materials talk to the guys at FF; they know their fabrics and will hook you up.