How accurate are bathroom scales that display body fat and do low carb diets effect this?

Most bathroom scales that provide body fat estimates do so using BIA (bio-electrical impedance analysis). The electrical impedance of body tissues is measured, which provides an estimate of total body water (TBW). Fat-free mass (FFM) and body fat is derived from TBW. Weight, height, gender and age are also required for this calculation.

The NIH found that body fat estimates using BIA can vary as widely as 10% depending on hydration status, consumption of food and beverages, ambient air and skin temperature, recent physical activity and the calculation used (http://consensus.nih.gov/ta/015/015_intro.htm)

Low carbohydrate diets have a diuretic effect. Water consumption is essential to combat dehydration.

I would seem that, given the above, low carbohydrate diets effect the accuracy of body fat scales and that any reading is more likely to be useful in a comparative sense from one reading to the next rather than as an absolute determination of body fat. In addition, the relative readings would only be comparable if taken under the same conditions, e.g. Time of day, state of hydration & meals, recent exercise, rest etc.

What are people's experience with body fat scales?