While there may be more accurate methods for determining body composition than others- none are 100% accurate. Even hydrostatic weighing (in the water tank) and other technologically advanced methods have inaccuracies because they generate body composition percentages based on assumptions.
As far as the accuracy of bioelectrical impedance analysis scales (and hand held ones for that matter) are concerned, they really are best for generating a before-and-after body fat percentage comparison- as a couple posters have mentioned. Specifically with BIA, discrepancies in whether you consider yourself "active" or "normal" can generate a huge difference in results. As long as you are consistent with what settings you measure yourself with, then you can start to track whether you are actually changing around your body composition i.e. losing body fat, gaining muscle mass, increasing bone density, etc. Unfortunately, the only accurate method for determining body composition is actually weighing each component of your body, and you would have to be dead to separate all your fat mass from lean masses.
smart shoe guy and promiscuous racquet man- xisbum