I have the Somnus 15, but and after trying it out for 8 months, I figured I would write a review, however, I couldn't find it on Backcountry's website... hmmm.
Anyways, the bag is awesome. I has a really nice silky feel to it. The Pertex shell feels really nice - like silk. Also, with 850 fill, the Somnus packs "down" really well and, for the 15 degree bag, it has over 5 inches of loft! That's Western Mountaineering status at Half the price, if you can catch the Somnus on sale. Also, it's just as light as some of WM bags too, which is insane considering its price.
Many people have said they have issues with the center zip hanging in their faces, but there is a zipper "guard" at the top of the zipper that you can tuck it into. Also, there's a small-ish baffle running down the zipper - no neck baffle though =(. The one thing that bugs me about the center zip is that the sleeping bag is less configurable, meaning, you can move down from the bottom of the bag to the top of the bag. Although I haven't had the need to do so, it would be nice knowing I could.
Also, I've noticed that some have hit this bag for it's inaccurate temperature rating. However, you must remember that the temperature rating is not for comfort for everyone - it is the comfort level of an "ideal" man. If I want to wake up hot in a bag, I slap on around 13 degrees to the rating. So to wake up warm, I add around 5 degrees to the bag. Also, I suppose each person is different so I suggest understanding if you sleep warm or cold. As far as the Somnus 15, I've taken it down to 21 degrees and been comfortable.
The compression bag is Horrible!! It does its job, but don't use it unless you want an extra 5 oz on your back. Just get a 1oz sil nylon stuff sack. However, that doesn't pertain to the sleeping bag.
Also, I've never used the foot box water bottle holder... cool feature, though! I can see how it would come in handy if temps dropped into the single digits.
Anyways, Here's what I like about it in short:
Light - light durable shell and 850 fill
Inexpensive - compared to what you would pay for a bag with these specs
Comfortable - silky smooth
Durable Construction - nice zipper, no snags. Great "seals"
Roomy - at least for my 5' 9", 140lb frame
Warm - I believe I sleep a tad colder than EN Ratings
Guaranteed Satisfaction - Backcountry.com, in general
When looking for a sleeping bag I considered 5 things. Here they are for the Somnus 15:
Weight (lower weight = better): 5/5 stars
Bulk (Lower bulk when compressed = better): 4/5 stars
Comfort (shell material): 5/5 stars
EN Tested (yes/no): 5/5 stars
Price (lower price = better): 6/5 stars
Go Stoic!! Another awesome product!!!