I have used Marmot down bags for over 12 years with happy nights in all conditions. I have used my Pinnacle 0º is conditions ranging from 8º all the way up to 45º. That bag just regulates heat so well. When it came time to fill my 15-20º slot again, I had a hard time deciding between the Helium and a couple of the 20º bags by Western Mountaineering.

In recent years I have become a stickler for weight, almost to an anal level, weighing each item and trimming weight where I can. In the end I went for the Helium, based on the incredible light weight, deep fill power, loft and hand of the outer fabric. Not to mention the obvious attention to perfectionist detail. This bag straight up kicks ass! I had heard from various friends that their WM and Feathered Friends bags had shed quite a bit of down over the years and I have not had that problem with Marmot.

First outing, I used my Helium on a 3 day trek around Mt. Hood with night temps dipping into the upper 30's. I didn't really put it to the test in that regard, but in comfort, roominess and carry weight I could not have been happier. My dog even nuzzled into the side of my bag at night and being a shepard akita mix, her coat keeps her plenty warm. She just loved the feel of the bag as much a me.

On technical aspects, the hood is the best I have ever used. No velcro to scratch your cheeks and it just wrapped my head in a big lofty cloud of down. The 1/2 zipper was a concern at first, but I rarely unzip my bags beyong that point anyway unless I used them as a comforter in my surf van.

My buddy bought the Hydrogen 30º bag and used it on the same trip. He is a leggy sleeper and the 1/2 zip didn't bother him a bit and he stayed very warm even though the temps approached the bags rating.

Bottom line, I would trust this bag in conditions much colder than the rating. Marmot is modest in its ratings, so I plan to give the Helium a try in some wintry conditions. It will be the ultimate lightweight backcountry snowboarding bag where weight is such a factor.

I would buy this bag if you are looking for an ultra lightweight, high-quality, down bag. Its pricey but well worth the peace of mind when heading into the mountains.