When the bio-impedience scales first came out ( I believe that is the type you are discussing) I was told hydration levels can throw them off. So I have always questioned them but do think they could be useful if you were consistent with your hydration levels for the most part. They may be a lot more accurate now then they were also.
My body fat % is quite pathetic right now and slowly working on it. Probably close if not over 25%. I have pics in my thread 38, Tired and Frustrated - #130 by rise80.