I had good luck last winter shooting in January in the Altai Mts (Mongolia/China/Russia border). I successfully shot down to at least -30c. I keep a small heat pack in the lid of the camera bag and keep spare batteries in inner pockets - the GH2 worked well. The camera must stay outside until you are done shooting, then put in a sealed (plastic) bag so it warms with out condensation. I was able to keep it out all day and used 1-3 batteries depending on amount of shooting. I actually prefer the small batteries as you can keep them warm easily until needed. Cameras don't get wet in cold weather.
The plastic bag trick only work if it's dry outside and you wrap some insulation around the whole thing before letting it warm up slowly inside. Where I live, winter days are usually humid, I use a hair dryer to warm up the gear, if you do it quickly the condensation wont have time to form, but it's not ideal. Kinda scary sometimes when you see the condensation form inside the lens while you do it. Don't professional use camera warmers?