So far its pretty fuel efficient. But now winter is here, and I have a feeling the stone is gonna be a big heat sink. The most propane I used was smoking the two turkeys, which I used about 1/2 of a twenty lb tank in about 6 hours. Then I was running 250 degrees plus. (I normally only run 170 degrees) It took me about two and one half hours to get it to 250, and the outside temp was about 50 degrees. WHen those stones get really cold, I think I will have a hard time getting the smoke house up to high temps quickly, but when I do I know that she will be stable, as the stone will hold the heat.
Getting back to the multi burner Northern tools unit. I am really not impressed with it, as far as the simple single northern burner. I think this is a real work horse. I am using that with my el cheapo Brinkman. I am using Adjustable regulators for both the smokehouse and the smoker. Word to the wise! Be extreamly careful when using adjustable regulators. They will go out if you turn them down too much. That doesn't mean that they will not put out any raw propane in your smoker. This could be a dangerous situation with the combination of smoldering wood in the smoker. I use a Maverick ET 73, and set the low smoker temp alarm regulary during the smoke process. I also check the flame regulary thru the process. If you do all this then you should be safe.
Remember this is a hobby, lets make it safe and fun.