I have one on the car, it regularly goes below zero. I think some pixels have gone in the screen (I don't look much). Sometimes it needs a reset (about once a month), which seems to vary with the cold. It is directly connected to the car by the dock and lives behind the glove compartment (controlled by the steering wheel controls).
If I need at some stage to get a battery, I noticed that my local electronics shop has them on the racks for not much money, but I doubt it will ever matter to me.