I was initially torn between these and the OR Alti's - lack of the ORs in my size led me to purchasing these (in an XS). These things are beastly - the XS is about the size of my torso, no lie. They fit nicely into the nalgene mesh pocket in my pack, though.
Let me tell you, these suckers are WARM. My hands are cold hands and the temperature rating on the packaging (-20 to -40F) does not lie. In close to 0F weather, I ended up hiking about in just the liners (and some other next-to-skin liner gloves) for a time, only donning the outers on my descent when the wind picked up.
Dexterity is as you'd expect for a big pair of mitts: you won't be performing any brain surgery with these things on.
The liner that comes with the glove is insulated, has a pointer-finger lobster (good for grabbing ice axes), and is waterproof enough to roll around in armpit-deep snow for a few hours without soaking through.
The outers are even warmer, and also waterproof. I plan on treating the leather palm as soon as I start to see wear, but that has yet to happen. The insulation continues into the gauntlet, keeping wrists warm. These aren't too difficult to get on, even with stiff fingers.
In closing, I wish that they were bright orange instead of black (partially why I wanted the Altis in the first place - red!) but embodied everything I've come to expect from BD and of a pair of serious mountaineering mitts. Excellent for those brutally cold days, potentially overkill for anything above 15F.
If your looking for warmth, I have found similiar performance between the Alti mitt and these. The Alti mitt is probably a little warmer, but they suck for dexterity. The lobster finger on the BD mitt makes it a little colder, but you can deal with you gear much easier, thus not pulling you hand out of your mitten, which deeats the whole purpose. Super great for those ski expeditions when you need dexterity and warmth
These mitts are huge. If the name of the game is to balance dexterity with warmth, my concern is that these have gone too far in the direction of bulk. I don't doubt they'll keep your fingers toasty, but it seems to come at the expense of being able to use your hands for much of anything. If you buy them, size down! Or, alternatively, check out the Hestras.