Efficiency of the microwave oven
Sort of.

Microwave ovens aren't actually all that efficient at converting AC input power into microwave power. A lot of input power is lost in the form of heat in the power supply and the magnetron. But pretty much all of the microwave power that is generated does go directly into heating the food &mdash; very little of it is lost elsewhere. And the coupling of microwaves to food molecules (primarily water and fats) is very strong &mdash; the heating happens within the food, rather than having to flow inward from the surface only.

Compare that to a conventional oven. The conversion of AC power to heat is 100% efficient, but a lot of that heat is lost to the environment rather than going into the food. Furthermore, since the food absorbs the heat only through its surface, it must be exposed to the heat for a much longer time. 

Therefore, the total input energy (power &times; time) required to heat a given amount of food is much higher with the conventional oven than with the microwave, but not for the reason stated in your first paragraph. (And note that you should never operate a microwave oven without food in it, because it will damage the magnetron.)