Right, this morning was the first sub-0c commute this winter. The forecast was -3C, feels like -6C, cross-head wind of 6-16mph.
I had the Giro drop handlebar mitts, Aldi winter cycling gloves and all of the kit I wear for the coldest commute (Buff head tube in balaclava mode, Aldi softshell,
Decathlon long sleeve, Aldi winter hike trousers, Sealskinz neoprene overshoes).
Hands? They weren't toasty but tolerable with gloves + mitts. I did sense the cold "pain" on my fingers after 10min but didn't get any worse for the rest of the ride (for another 65min or so) and my fingers were dextrous enough to use the keys and lock my bike in bare hands. No Reynaulds whitening!

Without the mitts, my fingers would have been in pain and would not have enjoyed the commute.
So, mitt is definitely a great addition to winter commuting. Absolutely ugly and un-aero, but I'm more precious about my fingers so I don't care.
Next thing... how to stop the rear brake cable from freezing up in the cold...