I commute year round, usually in the 20's F, but often lower, rarely down to 0 degrees. My own prefeence at less than 20 is a thin knit glove under a heavily insulated windproof shell. This year I'm going to try a lobster claw pair.

I would however like to suggest this additional piece of gear. Since I have a gap between my jacket and glove end, I always have to carefully tuck the wrist end of the gloves under the sleeve ends. Last year I made a pair of "wrist gaiters" by cutting the toe ends off a pair of athletic socks for my fingers and an additional hole for my thumb. The gaiters extend all the way up over my forearm. I have found that they keep my forearms quite warm, and usually become slightly damp with sweat, but not enough to chill. This may be subjective, but I think they help keep my hands warmer by keeping the blood flowing down to my hands warmer than otherwise without them.

This winter I will take note more carefully on those really challenging days, but last winter I didn't buy mittens, as I had contemplated the year before.