Samir S. wrote a review of Mountain Hardwear Masherbrum Mitten on March 2, 2011
These are amazing mittens. They're warm enough for standing around camp in sub-zero (F) temperatures wearing just a thin liner underneath. They're also durable so far, meaning I haven't managed to slice them open while using my ice axe yet. I have no idea how waterproof they are, but I can say that at the temperatures I've been using them at, there's no liquid water around anyway. I had to loan them to a friend while climbing Mt. Washington last weekend because his hands were freezing, and he had rave reviews as well. As far as dexterity goes, don't expect much...or anything really. I can hold a shovel fine, and I can grip my ice axe in self-arrest position...with a bit of trouble. But it's worth it if your hands are freezing; you can't have both amazing insulation and amazing dexterity at the same time.
As far as sizing goes, get the size you normally wear in gloves. I have long skinny finger, and I normally wear a large in gloves. I ordered both the large and xl to test, and with just a liner glove underneath (and that's probably all you'll ever need, unless you're going -40), the large fit perfectly while my hand swam in the xl. The insulation inside also compresses a tiny bit with use.