Big Agnes Mystic 15°
I've had the opportunity to test this bag at 12,800…
Fill: 775 fill down
Temperature Rating: 15 degrees F
Weight: 2 lb., 4 oz.
Price Paid: $200
I've had the opportunity to test this bag at 12,800 feet in Colorado's San Juans in August, among other places. The bag is exceptionally roomy for bigger guys (I'm 5-11, 215 lbs, size 46 chest--tried the Zirkel first but found it far too confining in the chest and shoulders).
The draft collar is nice and adds a little warmth. You must use a sleeping pad with this bag since it has no fill in the bottom half (I use BA's REM Insulated Air Core). This design is both good and bad. Good, in that it's almost like being in your bed at home under a blanket, and your bag won't slide off the pad during the night. Bad, in that the bag won't move with you if you change positions frequently (though it's so roomy that this isn't a big issue for me).
Also, I found that on very cold alpine nights (high teens/low 20's) I could detect cold spots where the bag joins the pad. There's supposed to be enough overlap to prevent this, but it's not perfect. This, combined with the low efficiency (due to its size), I think the 15-degree rating is a bit optimistic. It's more like a 20-25 degree bag, though I am an average-to-cold sleeper and like to be toasty.
That aside, it is pretty light, packs small, and is well-made. Overall, a good bag.