Small yes, efficient no. It's more or less on par with comparable minipcs in power draw, needs an active cooler and idles at over 3W. That's more than a Pi 2 uses going flat out. They keep increasing power draw with marginal efficiency gains and don't bother to do much power management. It's absolutely atrocious compared to the average smartphone SoC.

I really hope there's some kind of battery oriented low wattage high efficiency version planned someday, because we're up to requiring a 5A power supply and it's getting absurd.