I recently picked up a Marmot Hydrogen off of the REI outlet. I tested it this weekend, and it performed fantastic.
I was camping with scouts. The evening temp was about 15 degrees and the low was 11. Note, the bag is only rated to 30 degrees. I was using the following gear in additino to the bag:
Thin fleece balaclava
thin poly-pro base layer (tops and bottoms)
200 wgt fleece vest
100 wgt fleece pants
thick merino wool socks
9 mm Sorel boot liners.
And I was warm. I wear the same above even when I'm in my -20 bag. The tips of my toes got cold, but I want to attribute that to socks that were too tight. I'm grabbing a size larger next time. Just in case the bag failed, I had my -20 next to me. I never thought of getting it.
As for the bag itself, it is very wispy. It is 850 fp down, and it feels like it. It requires little effort to compress the bag. It came with a 6" x 12" stuff sack. It easily fits into this sack, and I can compress it with my hands into about a 6" cube. It is small.
In the morning however, I sensed that there was some moisture underneath the shell. This is most likely from sweat and insensible perspiration. I was only out for one night, so I cannot relay info about that. However, this test leads me to believe that a vapor barrier is necessary whenever temps make drying a bag difficult. I can't put an exact temp on it without more testing. I plan on picking one up soon and reporting back.
Also, this was the first weekend this season that I did not use my down coat. I wore a thin fleece jacket, a fleece vest and a wind jacket throughout the evening. I was fine, moving and never needed to reach for the puffy insulation.