I haven't found good gloves yet. I commute down to about -6C (about 21F) and even with good winter ski mitts that work fine on the ski hill, or walking in any temperature, my fingers still get cold by the time I get to work on a bike. I keep a pair of old socks in my trunk bag and put them over the fingers of the gloves and that sort of works, except for my thumb. I think a major problem is that metal brake levers suck the warmth out of your gloves.

I have some nylon lobster claws with minimal insulation and they are ok down to about 5C (40F).