Well, for me, I got an insert because I already had a fireplace which, as we all know, is useless for heating. The room is not huge, so for me an insert is the way to go.
As far as efficiency, on one hand I suppose it CAN'T be as efficient (law of physics can't be denied..) but on the other hand I wonder if its that much different. I mean, radiant heat only goes until it reaches a solid object and then it turns into heat. A good example of this is a heat shield below a free standing stove or a heat shield above a stove/below a mantle. In both cases the air space obstacle to the transfer of heat.
So, is it possible that the radiant heat of the stove (inside wall) radiates until it hits the jacket (outside wall) and then turns into heat, which is then "carried" into the room by the airstream produced by the fan? (the question mark is because I don't know. Furthermore, there is another airspace between the outer jacket and the masonry.
One final thought about inserts: Yes it IS dirty. Dust, especially, is an issue. I'm probably going to try one of the small, portable, hand held vacuums.