I've never used it, for exactly the reasons you gave, and I have lost over 100 pounds. I know what I can eat in terms of carbs -- pretty much unlimited lowcarb vegetables, one or at most two servings of lowcarb fruit(usually berries, sometimes melon) a day, reasonable amounts of cheese, small amounts of nuts, and that is pretty much it. I don't need Fitday to help me keep track of that. When I first started out, I did spend some time studying charts of carb counts and that helped me figure out which foods were likely to be problematical.

I have no idea how many calories I eat each day or what percentage of my food is protein/fat/carbohydrates. I try to listen to my body and eat when I am hungry and stop eating the second I am full. It works for me.