How much "juice" the heater takes: My first mostly-freeway commute to work (12.5 miles) I averaged 318 Wh/mile, even though I kept my speed moderate (66-ish mph). This commute is slightly hilly and the destination is 200-300 feet higher in altitude than the point of origination, but that number still seemed high.
I later realized I'd had the heat on, and made it a point to turn the climate control system completely off for subsequent commutes.
The results: Same route, same speed, roughly 240 Wh/mile.
Other manufacturers use heat pumps for heating - I wish Tesla would do that. It's great that the Tesla heater can heat so effectively, but a heat pump would potentially deliver the same amount of heat with less than a third the energy consumption. Given that they already have an AC (which is basically a heat pump hardwired for cooling), this seems like an obvious enhancement.