How efficient are masonry heaters?

A masonry heater burns wood unlike any other appliance. Rather than using the fire it's self to heat your home, a masonry heater uses a short, hot fire to charge the thermal mass of the stove. It is the thermal mass of the stove that delivers warmth into your home for 12 hours or more after the fire is out.
A properly designed masonry heater burns with an appropriate oxygen supply and at a very high temperature. This allows the masonry heater to burn all the volatile gasses that form creosote in conventional wood burning stoves, giving you more heat from every stick of wood you put in!
The hot exhaust is then run through a heat exchanger, where most of the energy contained within it is harvested by the stove's masonry mass by the passive process of heat transfer, until the gasses have cooled sufficiently to be exhausted into the chimney.
A short, hot fire charges the stove with energy quickly, and the stove lets that energy slowly and evenly into the space being heated.