I drove my father's Miata last winter here, it was in the 30s with the occasional dip into the 20s. It really wasn't bad, the heater blasts HOT air after just a few minutes trundling around. I will say it becomes tail happy as the summer tires become hard as a rock and lose their grip, but you just slow it down a bit and don't hoon it as much. There's some guy that famously drove it around town in the snow with the top down because the heater was good enough.