Omron Full Body Sensor Composition Monitor and Scale

I have one like this at home. It is entertaining, but it gets its reading via an impedance test. The readings will vary quite a lot from day to day, and even from moment to moment depending on how wet/dry your feet are.

If you use it at the same time every day, and under the same conditions, it can give you a general idea if your fat percentage is going down, but it isn't anything you would want to use with confidence.

I use mine right after I shower so the bottoms of my feet are still moist. It helps the conductivity. But just to give you an idea of how much it can vary, I get a reading of 24.6% fat when my feet are wet, and a reading of 34.8% when my feet are dry. Obviously the amount of fat in my body did not change between readings.