Aside from being named after a gas-guzzling redneck pickup truck, the Arc'Teryx Dually belay parka is one of the best designed mountaineering tops available. The lightweight pound and a half Dually uses their ThermaTek insulation inside a high-gloss, slightly crinkley DWR shell.
ThermaTek is a synthetic insulation that bills itself as being 100% hydrophobic due to each individual fiber being DWR treated. In practice it does a great job of maintaining warmth while wet, critical when your rope gun partner is raining hunks of ice on you while you're sitting in a wet and cold belay station. It also compresses at least as well as any other synthetic on the market and maybe just a tad better.
Nice touches include a snap at the hem so you can unzip the 2-way zipper a bit from the bottom and have a nice little hole for your belay device without any loose ends flapping around. Every alpine jacket should have this feature. The Dually has two extra-large stretchy pockets inside that easily swallow quart bottles, another big plus especially on summit day when you need all the integrated storage you can get. There are two external zippered handwarmer pockets that for some reason have a microfleece lining that feels great until it gets wet. The hood is helmet compatible, the zipper flap is a nice stout design that flares at the top and there's a small but functional stretchy draft collar inside. Note: no chest pockets, which is fine because if you're wearing three layers chances are you've got chest pockets on every one of them already.
The Dually is a great synthetic parka no doubt. I run cold most of the time and the Dually was comfortable down to about -10F with a medium weight layer underneath. The sizing is different from the normal tailored cut of Arc'Teryx clothing: it's cut large to fit over your other layers. You might be able to go a size down and still have room for layers. The Dually is an excellent choice when you will be dealing with extreme cold and wet during the same epic.