One thing I have learnt over the years is that as far as photographers are concerned (like camera bags!) there is no such thing as the perfect pair of gloves where warmth and dexterity combine. For really cold environments, I use Merino liners (sometimes with a gel hand warmer inside) and mittens, but for day-to-day, I wear a normal pair of Thinsulate gloves which I take on and off with these hand warmers switched on low power in my jacket pockets.
There are three power settings and I find that even when on the lowest, they provide sufficient heat for that welcome boost and, without switching on and off, pretty much last all day. I really have found them indispensable during cold days - and nights - in the field and, for the price, consider them extremely good value. Details, here.
