You'd be best helped by using the search button up top... there have been 2 or 3 threads on this in just the past couple months!
But just to summarize - they're not accurate at all. The only truly accurate test of BF % is a 7 or 9 site caliper test.
The scale's only real benefit is tracking progress. i.e., you might get a reading of 20% BF from the scale (even if its not accurate), but any changes in that reading should remain consistent.