Big Agnes System bags with DownTek insulation offer a roomy, rectangle-shaped sleeping space with an integrated Flex Pad Sleeve on the bottom. With technical fabrics and construction, compressible insulation and tons of comfort features, these comfortable sleeping bags are designed to be lightweight enough for backpacking but cozy enough you'll want them in the campground too. The streamlined silhouette and Flow construction increase the thermal efficiency keeping you warm from head to toe. The Flex Pad Sleeve unites easily with your pad, and as you change positions over the course of the night, will keep you from rolling off your pad.
Not even close to a 15 degree bag, I used this with the Q core insulated pad and was freezing with clothes and a micropuff on all trip in ~ 25* weather. Last weekend I gave it another try in around 35* degree weather and still wasn't comfortable. This is a decent bag but the temp rating is wayyy off.
I like everything about this system with exception of one thing. I purchase both the bag and pad. I love the fact the pad attaches to the sleeping bag. Allows me to move around without the bag. However, I truly hate the no insulation between the bag and the sleeping bag. What ever part of my body that is on the pad is always cold. I have there warmest sleeping pad but it is not good enough. I would likely not buy the system for this reason. Since I already bought it, I bring a fleece liner and wrap the pad with the liner. That seems to work fine but adds another 2 lbs.
I backpack in Arkansas, it can be 110degrees f in the summers and i went out in the winter when it 17 degrees f and with my 4.5 r value big angus mat and this bag was on a foot of snow and was warm, the summer the same gear is light enough I just lay on it with maybe my feet in. Couldn't be happier!