I have the inner and outer bag and use them in all sorts of conditions. Even though I am not of a small size they fit very comfortably for me. I move a lot around in my sleep and toss and turn a lot.
I am also a warm sleeper and find the use of the arm zips very handy for keeping me in the right place and cool enough to get a good sleep and only need to close them up in the coldest of conditions normally around -10 till I take my arms in the outer bag but keep them out of the inner bag, once it get below -15 then i may consider putting them in on both bags.
I use them in various conditions with other equipment as well such as Jervens Bags, Bivi bags, roll mats, swags, and even hammocks. The major advantage for me is the capability to use my arms but still be in the bags all snuggly and be able to put a brew on. because you can use your arms they also work well in hammocks in colder climates and because the filling is not a loft based one but a fleece based one it works very well for overall coverage and find I get very few cold spots like most sleep bags in hammocks.
I normally only use a poncho liner as my under-quilt as it packs small and for hammocks the main thing you are fighting is convection cooling effect and in winter I use a Jervens bag over the top with a couple of poles over the top. The in my buffalo bag/bags and I am good down to below -20 C in Scandinavia.
looking at the bulk i find it better to store the bags separately rather than one inside each other and pack them loose around the rest of my kit inside my Bergen/rucksack they works very well for all sleeping bags but especially for the buffalo bags and because of the pile and pertex constructions they last a lot long the all my synthetic and down fill bags my buffalo bags date for the 80's and are still going strong over 40 years later. Not the smallest or lightest but they work and stand the test of time. same thing you find with all buffalo products. The tend to get borrowed or taken long before they wear out.