I live in central Minnesota and try to commute semi-regular through the winter. I have bar mitts for my road bike (with drops) and another set for my fat bike (flat). I throw them on for most rides below 35 degrees as my hands have poor circulation. 30-35 degrees I will use bar mitts with thin gloves, below 30 I use lobster gloves with the bar mitts and all is well with the world. ~5 deg and under I can make it 30-40 minutes with the bar mitts and lobsters and my fingers will start to get cold.
They do have drawbacks to consider! If you can deal with lobster gloves or mittens alone, I would recommend going without bar mitts. Bar mitts are not fun in a cross wind. Bar mitts make it harder to get your hands on/off the handle bars (think turn signaling).