They are 80% reliable... If you need an exact measurment it's best to have your body fat measured and then make the adjustment to the one on the scale...
but one thing for sure the percentage of fat number on the scale will go
down as you loose FAT! Then when/if you occassionally gaining weight because you are gaining MUSCLE (or it's TOM!) you will not give up and end up gaining real weight, by which I mean weight from FAT!