These fries are so GOOD. Haven't tried them with any other oils yet and Lord willing to provide a consistent source of tallow I won't have to. These are not very greasy either. I've noticed with the tallow that the oil level hasn't decreased in several uses the way that peanut oil and/or tallow often does. It seems to cook much cleaner.
Do you think a deep fryer extends the life of the oil? I'm thinking the consistent temp would help. I have to really watch when I'm deep frying over the stove. Oil temp changes so quickly and then my expensive oil seems ruined. 
I would say, probably 'yes'. I tried my hand at deep frying on the stove and it's very difficult to get a consistent temp. The fryer, I have to heat slow until the tallow is liquid or it tends to smoke alot, but it's a fairly quick 2-step process. I turn up the heat to the very lowest setting (I'm guessing 100, but there's not marker) and once it's all melted, turn it up to it's highest settinig (375) and toss in the food and WOW! this stuff is awesome.
With my old fryer, I never heated in two steps (however, never tried tallow - either peanut oil or lard) and I threw in wet food, frozen food, etc. The lack of prep on the food may also make a difference, but I never tested the theory. I started new practices with a new oil. I did try cooking frozen wings on the stove before I got the fryer and the results weren't good. The wings took longer to cook than my old fryer and they were very dry as a result.
