I've worn these mitts quite a bit, but, like Will, I've carried them for many more miles than I've actually used them. I want to point out three things:
1- I can't emphasize enough the importance of sealing the seams with SilNet. This is not easily done, but it needs to be done perfectly in order to make them waterproof. Without sealed seams, they are not waterproof, and your hands will get wet. And having cold and wet hands is miserable.
2- In addition to using them for backpacking, I run with them often in the winter in order to add warmth to my standard running gloves (DeFeet Duraglove) and to shed any precip that may be coming down. I've worn them in temps between 15 and 35.
3- When backpacking the mitts are part of my 3-season handwear system, the other part of which is usually the DeFeet Duragloves. When morning/evening temps are regularly less than 30 degrees, I prefer bringing the BPL Vapor Mitts made by RBH Designs, which are much warmer (in both dry and wet conditions) and only slightly heavier than the Mitts/Duragloves combo.