We've left the Sub Zero's review intact below. Note that it is now highly outdated and likely inaccurate.
If you need a serious jacket for extreme cold, this is the one. The Mountain HardWear Sub Zero SL wins our Recommended award because of the many functional features that make this a desirable jacket for mountaineering, such as a longer fit, a large hood, very water resistant material, and clever ways of sealing out wind and keeping in warmth. However, that means that this jacket is not very lightweight and does not stuff down into a compact sack like all the other jackets in this review. This is also why it does not receive the Editors' Choice award like the lighter and more versatile North Face Catalyst. If you are looking for a jacket for everyday use in the winter, consider The North Face Nuptse or Aconcagua jackets, which are still warm but less heavy duty and much less expensive. If you need a lightweight and packable jacket, compare the other alpine down jackets: Mountain HardWear Nitrous Jacket, Patagonia Down Sweater, or the MontBell Alpine Light Down Parka.
This is one of the warmest and most water resistant down parkas in our review. It is a great option for those looking for a fully-featured, heavy duty jacket. It features fleece-lined pockets, an internal water bottle zippered pockets, and its own stuff sack. It uses Mountain Hardwear's proprietary waterproof material. However, the jacket itself isn't waterproof because it isn't seam taped. All these features make it one of the bulkiest down jackets we tested. It is an excellent choice for colder trips. The weather resistant shell fabric makes it a great choice for Cascade volcano climbing and New England ice climbing.