I use a Feathered Friends Vireo. It's a zipperless, hoodless bag with a fairly narrow, well insulated foot section and a relatively loose fitting, less insulated upper section. Mine's made of Pertex Quantum and filled with 800+ FP down. I love it. It's designed to be used with a poofy jacket and a hat/balaclava/etc, both of which I'd be carrying anyways. It's easy to adapt to different temperature expectations by choosing a different jacket or hat. I use a Montbell Thermawrap UL, Patagonia Micropuff, or down FF jacket depending on how cold I expect conditions to be. My hat choices range from a simple watch cap to the Jacks R Better down hood. I prefer to carry a synthetic jacket for "insurance" purposes.
In a lot of ways, using a Vireo is similar to using a down quilt with a jacket. Both have plenty of room to fit a nice warm jacket underneath without compromising the loft of either the jacket or the bag/quilt. Both options can save a chunk of weight. The main difference that I can see is the matter of draftiness. Quilts are draftier, but that's not necessarily a bad thing. Lacking a zipper, you have to be a bit more creative if you're feeling hot. I have been able to manage fine just by loosening the drawcord on the Vireo and maybe sliding up a bit to reduce the jacket/bag overlap (and move my feet closer to the less insulated portion of the bag. But if you have a hot foot condition, this isn't the bag for you.
I've been tinkering with the idea of getting a Jacks R Better No Sniveller quilt to try out. It's very flexible and reasonably priced (not mention that it was the only "bag" in the article referenced above that rated higher than the Vireo) plus I already have the matching hood. I also think that the quilt would work reasonably well as an overbag for the Viero in cold weather.
Anyways, to make an overlong story longer, I like my Vireo a lot.