If you mean the Tanita body fat scale - well...my personal opinion (as well as many others at
www.leanandstrong.com) is that while the scale weight is accurate, the body fat measurement is much less so (someone at L&S experimented and found that their phone book had 6% body fat according to the Tanita!). The thing is - if you have your bodyfat professionally measured with a bioelectrical impedence (the same principle that the Tanita uses) the technician/trainer will put electrodes on both the top AND bottom half of your body, so that the impulse will go through your entire body and thus give a more accurate reading than the Tanita. I've read somewhere (it's been awhile so I'll have to look for the source) that the electrical impulse from the Tanita bodyfat scale will travel, perhaps, to waist level at most, but there's no way it can travel all the way through your body. (I believe that's why so many women's bodyfat percentages are so high using those scales; after all most of us carry the majority of our fat on our hips/thighs).
And those hand-held devices have the same problem - I believe they are accurate as far as your arms and perhaps the upper torso...but if they were powerful enough to get an accurate full-body reading, you'd probably feel a shock!
My advice - have someone who KNOWS WHAT THEY ARE DOING measure your bodyfat. Most technicians that use one method (Bio-electrical impedence, Bod Pod, immersion) will also back that up with a 5-site caliper measurement.
Personally - I just go by the Pants-O-Meter - clothes fit. IMO that's the ultimate measurement and is totally accurate!