Compressibility
The Mobile Mummy 800 15 takes a little more than average effort to compress into a compression sack, which is not surprising given its excellent loft. It also does not compress into an overly efficient size and is among the largest compared to other bags in this category.

Warmth
With its 800-fill down (featuring DriDown, treated down to repel moisture, improve loft and dry faster) and tight design, the Mobile Mummy is a very warm bag (less so when the arms are left outside the body—which for some users was the only way they could sleep more comfortably given the constrictive nature of the bag). Warmth therefore comes at the expense of comfort. 

Features
The Mobile Mummy 800 is a garment-style bag with a unique middle zipper and self-closing arm holes. This makes it easy to sit up in your tent without getting out of your sleeping bag. “Finding” the holes took a little getting used to and, being slightly claustrophic, it was not a pleasant experience. Other testers had a similar experience. On the other hand, the self-closing nature of the holes (making your arms a little harder to get through) is also what prevents heat from escaping the bag while your arms are sticking out the holes.

Hooks on the bottom of the bag allow for the footbox to be stowed for those who wish to walk around camp. While Sierra Designs advertises the fact that you can stay in your sleeping bag and stow the foot box if responding to nature’s call during the night, this is simply far fetched especially for women.  To stow the foot box you need to stand up, and to fiddle with the hooks to accomplish stowing in dim light and a groggy state is impractical at best and downright annoying at worst. I suspect given the trouble there are users who will elect not to stow the foot box and will consequently drag their bag around camp, resulting in durability and cleanliness issues.

The jacket-style hood on the Mobile Mummy is also unique. Where other bags will have cinch-cords that allow users to customize hood fit, Sierra Designs chose to create a one-size, jacket-style, tight fitting hood with no cinch cords. This is a plus or a minus depending on whether you want that feature – a minus for me as I do not find it comfortable to sleep with a tight fitting hood that zips up to my mouth (even if I leave it unzipped it is still tight fitting) but other reviewers did not mind this feature (but were neutral on it, not preferring over a traditional sleeping bag hood).

Overall Comfort
Although the bag has beautiful loft and is warm, because it is designed to move with its user, it is extremely tight fitting with a tight foot box. For those who enjoy cocoon-like enclosures, the Mobile Mummy may be for them. For those who do not, testers in this test included, they will not find it comfortable. 

Durability
The Mobile Mummy 800 15 held up to wear and tear and showed no marks of use in the testing phase. If the bag is, as contemplated by Sierra Designs, worn regularly outside camp, life may be shortened.