I purchased the Mountain Hardware Sub Zero SL Parka specifically for a high-altitude mountain climb (Aconcagua) and although it was warm enough, it did not perform as expected. I wore it briefly a few times at Camp 1 and Camp 2, and all day on Feb 12, 2008 (summit day).

Although it was quite warm, poor design/ construction revealed itself in several very disappointing / annoying ways:
1) The main zipper was very hard to get started. It required a lot of struggling to get the zipper started. In the early morning of summit day, I had to remove my outer gloves and fiddle for a long time with just thin polypro liner gloves in extremely cold conditions before I could get the zipper started properly. Plus the zipper is metal. This seems like a good recipe for frostbite. An unreliable main zipper is a fatal flaw for a climber’s parka
2) During the climb, the zips on two or three of the pockets snagged on the fabric backing that runs along the zipper just due to normal operation (opening and closing the pockets). It was too cold to mess with them, so I left them in the snagged / half-closed state until I returned to high camp. I later found that I had to operate the zippers very slowly to prevent snagging
3) The removable hood needs a different, more secure attachment system. At one point after a brief rest stop, I put my pack back on and realized that the hood was trapped between my pack and my back. I pulled on the hood and it started to detach, being held only by a few snaps.
In summary, the jacket was a hassle the entire day. I think it needs significant revisions to meet the needs of high-altitude mountain climbing. It is less expensive than some other parkas, such as the North Face one, but in retrospect, the savings were not worth it. I returned this item.