The question is whether or not they actually still do the job properly in subzero weather.

Whether driving or idling, the thermostat does indeed greatly restrict the coolant flow to the point that virtually no coolant goes through the radiator until the engine reaches operating temperature.

However, an internal combustion engine is only about 15% efficient, in that of the thermal energy contained in the gasoline, about 85% of it is dissipated as waste heat rather than as energy used to propel the car. Driving the car you are using at least four times as much fuel per unit time as you would use idling. That means you are generating about three times as much waste heat. With three or four times as much heat being generated, the engine will warm up three or four times as fast.

Under extreme conditions (minus 40 degrees C or F and colder) some engines (particularly small ones where cube/square law becomes important and with alloy blocks and heads that radiate heat more efficiently) simply will never reach operating temperature while idling. You can drive home, leave the car idling and see the engine temperature drop as you watch the gauge.

tanstaafl.

"There Ain't No Such Thing As A Free Lunch"