I'm currently using a pair of Outdoor Research Snowline mittens with poly-pro glove liners underneath. Rode this morning at 35°F/2°C with no issues. I have poor circulation to my hands though and need about twice as much insulation as most people apparently. Last year I rode the same liners under Pearl Izumi AmFibs and almost puked after rides when it was below freezing my hands hurt so bad once I got inside. They were good for 15 minutes; I was miserable the last 45 of my commute. I've ridden when it's down as low as 16°F/-9°C and am looking at wool mittens to wear instead of the poly-pro liners for those days.