All the feedback here is correct. To summarize:
Bioelectric impedance depends on hydration levels and assume you to be fully hydrated.
There are 2 settings (usually): athlete and general population. These are 2 different equations. In addition, the same reason why BMI is not a good indicator to monitor obesity. I haven't yet seen a study where they characterize the people that they use from which they create the equations. In all likelihood, they can just be people with lower muscle mass (yet still lean), thus the weight component still plays a role, along with height, in the equation, even though they are also measuring electrical impedance. All are taken into account when they develop the equations. Also, they include age because as we age, the normal person increases their internal fat (fat around organs in other tissue). So, that also throws off the reading as it's measuring more than just your subcutaneous fat, the fat between the skin and muscle hiding all those hot six packs!
Also, the hand held ones and the scales have different equations. Both are just estimates.
The advantage is that it will give you the same reading (under the same conditions--like full hydration): it's repeatable, but not reliable/accurate.
Calipers are a great estimate, when done properly. Thus, these are reliable, but unless you are an expert at using them, sometimes not very repeatedable and challenging to do on yourself.
Dunk tanks: well, these are great, but expensive and acess is limited.
They even have MRI like scanners now that can scan the whole body, but again: expensive and acess is limited.
Now, it's great to set goals, but really what does the number give you, except bragging rights?
For the sake of goal setting, I suggest measuring your waist size and track that. Take the measurement at the same place each time. I even take 3 measurements along the ambodmen when I track my clients, as people store fat differently along their core. It's easy, reliable and repeatedable.
Just food for thought.