Rating: 5 / 5
I now have a number of seasons with this bag and can now pass considered judgement. I have a long version of the bag when Marmot was claiming down of 900 lofting power (current models use 850) although I can tell no difference between the quality of down used between this and what Western Mtneering uses in it's premium bags.
The Hydrogen is a slim, mean half zippered mummy bag with a superb hood and muff that are comfortable and efficient in sealing out drafts. The stiffened draft tube does a good job of allowing snag free zipper operation. The Pertex Quantum shell is pretty downproof and has a very water repellant DWR treatment. Total weight w/o stuff sack for the long size (up to 6'6") is 23.6oz., 13 oz. of 900 down fill, 4.3 in. total loft, single layer loft of approx. 2.5 in. This is a technical bag made to keep things to essentials---it's not for someone who requires lots of wiggle room, although it will accomadate side-sleepers.
The user has some additional temp. control by shifting down around on the upper portion of the bag due to the continuous baffles there. Below the zipper there is a cross-block baffle to control down distribution. The footbox has a nice efficient size (but not too tight for larger feet) and shape. The bag is rated by Marmot at 30 degrees F and is probably not an overly enthusiatic rating for most users in good shape. Tarp camping with a light bivy, I have had this down in the low to mid 20's and to even lower temps. wearing insulated clothing to bed. The bag is well made as are most of Marmot's high end bags, of a level similar to WM. Retail is about $330 (ouch!) but possible to find at lower prices with judicious shopping research.
Noting it's slim cut, I recommend this bag for ultralight summer season high country use, particularly for those new to UL backpacking and anyone who is uncomfortable with the idea of quilt sleeping (and their learning curve for lower temp. use) and are not aspiring for 5 lb. baseweights.