For roast potatoes, it’s amazing. Fondant potatoes, I’d imagine it is similarly better than butter or oil. However, it would be wasted on a regular stir fry, for instance.

The other reply pretty much sums it up. Use it in lieu of butter, not vegetable oil.

Any instance you'd use schmaltz or beef tallow, duck fat can be used without issue. Though I know they deep fry potato wedges in the stuff in Frendh Canada... I just don't have that much fat at once to use it for deep frying haha

https://www.seriouseats.com/fondant-potatoes-recipe-5217320

This is the recipe I use, I replaced all the fats with duck fat during my last prep of it, but I think it would have been improved slightly by using an herbed compound butter (or margarine, in my case, I chose duck fat and duck bacon for making this with the beef bourguingon for kosher reasons) for the butter they throw in before the potatoes go in the oven.