How accurate is to measure your body fat with a scale?

I don´t want to measure my workout results just with my weight I prefer to measure the fat, what do you think? http://www.amazon.com/EatSmart-Precision-Digital-Bathroom-Technology/dp/B001KXZ808/ref=sr_1_4?ie=UTF8&qid=1311092236&sr=8-4

Public Comments

1. most of the time around 5 pounds off