I use the Alti Mitts in NE winters... mitts of this type are really only good for grasping the head of an axe or using poles with straps. They are too bulky for almost any other use. Given that, you will still need to wear a liner glove (powerstretch is my choice) as you need to remove the mitts in order to do anything (unzip a jacket, take off your hood, scratch your nose...).
My Altis are JUST warm enough for me when it gets below zero. They are XL, and my thumbs begin to get cold due to the slight tightness of the fit in that area. The insulation is simply too bulky in the thumb.
That said, they are a tough and versatile system. The outer shell is insulated, allowing you to wear a thicker glove instead of the bulky liner mitt when conditions allow. The shells make great "belay mitts" due to this.