I purchased this bag for my trip up Mt. Kilimanjaro. While is worked pretty well for most of the trek, we spent our seventh night in the crater (19k'+) and I gotta say, this bag didn't quite cut it at that alt/temp. I supplemented it with a liner that was supposed to add another -15 degrees and wore two sets of long capliene 3 underwear and fleece pants on top of that along with two pairs of socks. I was still shivering.
Something else to consider about this bag is that the draw cords around the head and neck are hard to loosen when you need to cool off. The little plastic guy gets lost in the bunched up fabric and you can't squeeze it easily with one hand. Two hands is not really an option because the bag fits so close around your shoulders, it is impossible to have your arms up like that.
On the positive side though, the bag repells moisture very effectively. Also, it is extremely light ad packs down small.
My girlfriend got the Mountain Hardwear -15 degrees Lamina and she was much warmer than I even though she is a cold sleeper and had no liner or extra clothing. Her bag always had more moisture on it than mine in the mornings though.

I bought this because I wanted a jacket that was waterproof and insulated but also light. It delivered on all counts. It is the most versatile piece of gear I own. While it would be a little too warm for say summer hiking in the Smokies, anywhere else I can leave the plain rainshell at home and just bring this one jacket to keep me both warm and dry.
If you get too cold, you can put a fleece on under it (I have a medium, and it firs just right, with or with out a fleece) and if you get too hot, just adjust the side vents. Those go practically down the whole length of your arm and body and feature 2 zippers so you can put the opening right where you want it.
I love that the hood is insulated too, although if you are wearing a hat, like my MH dome, then you can forget about hearing anything anyone says to you when the wind is blowing.
Another nice thing about this jacket is that you can put it in a compression sack (not included) and is will pack down smaller than a football.