It can be helpful if you understand their limitations. Although it may not accurately peg your actual body fat percentage, you will be able to see changes if it is decreasing. If it is the same scale and in your home and you weigh yourself under the same conditions (without exercise, good hydration, etc.), you would be able to see a trend line going downward if you were changing you body composition.
I have a BF scale and have had my BF comp done with calipers several times. For me, they correlated, so within the error margins of both of these instruments, my BF scale was pretty accurate for me. The only really accurate method is hydrostatic weighing. If you live near a training hospital or a college with a sports fitness department, you may be able to get a good deal.