In reply to Max factor:
It depends whether you have flats or drop handlebars. I use mitts on my flatbar-ed mtn bike in really cold weather and it's great.
Sti / Campag shifters on the hoods require more dexterity, and mitts tend to make them a little awkward. My cold weather touring bike has drops, but I have cross levers for the brakes and barend shifters so I can wear mitts.