I commuted through entire winter here in Wisconsin, down to temperatures of about 14 below (Fahrenheit -- and that's actual air temp, not wind chill rating, that was something like -35). Second the Bar Mitts (or a knock-off, what I think I actually have). Once you get below maybe teens they are almost necessity (especially if you are going 20mph+, that adds a heck of a "windchill" for your hands even if there is no wind). Underneath I had $70 goretex gloves. The bar mitts worked so well that frankly my hands were warmer when I got to work on that -14 ride than other days when I rode when it was +15-20 out but without them. I wanted to restrict using them to only the coldest rides because I was a little nervous about riding on ice while my hands were inside them; wasn't sure I would be able to brace a fall (wear a very good helmet if you do get some!) Frankly the far bigger deal in dealing with the coldest was eye protection, specifically keeping my riding goggles from fogging/frosting up.
BTW I've also been riding on Schwalbe Marathon+ since swapping out my Schwalbe Winter Marathon studded tires. No issues at all to report, seem like great "semi-slicks." Like 'em better than the stock Kendas I rode on the first couple months I had the bike, anyway.
