The efficency of a chimney depends on how air tight it is built. If your chimney has excess air infilteration this will allow the flue gases to cool before they reach the top outlet and you will get a build-up of creosote. If your chimney is somewhat air tight then the gases will stay hot to the top and you will have a very efficient heating source. It all depends on how the mason built the thing.
If your chimney allows excess air infilteration then the only real way to correct the problem in my opinion is to line it with a stainless steel liner. This will increase the draw, ensure the flue gases stay hot the whole way up the flue, and you will generate much less cresote, and more heat in the long run.
I brush my lined chimney every year and all I get out of it is about a 1/2 gallon of dust, and the chimney is almost 40 foot tall.
Craig