I always use a high fat Euro/Amish butter when cooking.
This is a very politically loaded question, and the answer you get will depend entirely on who you ask.
Personally, I recommend the chapter on Fats in the beginning of Nourishing Traditions by Sally Fallon. Fallon's position is that animal fats are a necessary source of nutrition for brain and immune function. However, she does address both sides of the argument and cites (most of) her sources. So it's a good chapter to read as an introduction to this issue.
Most of Fallon's argument relies on pre industrial diets, where people ate large amounts of animal fats but had very low incidence of heart problems and other Western diseases. Well worth a read, even if you are in the anti-animal-fat camp.
My personal position is a bit more moderate. I like to mix up my fats, go with what flavour I want rather than worry about the health benefits. A moderate amount of different kinds of fats is sure to give me a more complete nutritional profile, instead of a lot of just one fat. They all have their own nutritional benefits and detriments, so I try to seek a balance in my diet.
(my opinion) If you are already eating butter, replacing some of that with duck fat is unlikely to do harm (depending on other factors of course). Duck fat is easier to digest than a lot of other animal fasts, and tastes extremely yummy.
I agree with this statement overall. But... doesn't butter count as an animal fat, too, anyway?
Yes, butter is considered an animal fat. Fallon is a huge advocate of eating butter from grass fed animals, but suggests that one should include other animal fats in the diet as well. Personally, butter is okay, it has some good brain stuff in it that would take dedicated effort to get it strictly from vegetable sources. My only problem with butter is that it never really tastes good unless it's fresh. Cultured (butter made from fermented milk) is my personal favourite for health, taste and digestibility.
You might like to take a look at this chart from Eating Rules http://www.eatingrules.com/Cooking-Oil-Comparison-Chart_02-22-12.pdf On its heathy-to-unhealthy axis, it puts duck fat as a little healthier than butter from grass-fed cows and a lot healthier than butter from grain-fed ones. Duck fat has monosaturated fats, which increase "good" cholesterol.
Not much will happen to you except your food will be extra delicious. The natives of the Dordogne where duck fat is regularly used in lieu of all others, historically live to a ripe old age, and have a very low incidence of heart disease and stroke.
You won't have leftovers; you might finish a 4 serving dish all on your own.
At the end of the day: fat is fat. But, flavor is up for debate! Your food will be extra delicious and nearly impossible to save for later. Yummy! Take me to flavor town. You can also throw in some bacon fat. Gettin a little crazayyyy
I have used it in cookies with great success:
As a card carrying member of Team Duck Fat, I say use at will!