I've got one about 8 years old, and it does just fine down to 0°F (it hasn't gotten colder than that here). It doesn't even have any kind of auxiliary heat.

It's fine. The only difference when it's super-cold is that the air coming out of it isn't as warm, so the heating cycle stays on for a longer proportion of the time. But it keeps it 70°F inside no problem at all.