Skoorb--
I have one made by Tanita--cost me like $100 or so.
I've found that it can be pretty variable in its readings, but if you "weigh in" at a consistent point in the day every day you should get readings that are pretty accurate. In the manual it says:
"To get the most accurate absolute values, measurements should be taken while wearing no clothing, preferably in the evening, and after urinating. If you do not undress, always remove your socks, and be sure the soles of your feet are clean before stepping on the measuring platform. Stand erect on the platform. If used after bathing, the body must be completely dry.
It is best to take measurements at the same time each day, preferably in the evening. If doing so is not convenient, remember to wait about three hours after rising or eating, or hard exercise before taking measurements. While readings taken under other conditions may not have the same absolute values, they are accurate for determining the percentage of change as along as the readings are taken in a consistent manner. To monitor progress, compare weight and body fat percentage measurements taken under the same conditions over a period of time."
You also have to stay hydrated. If you aren't getting enough water and you step on the thing, it's going to tell you you're a fat-ass (even if you're not).
Bottom Line: They can give fairly accurate readings, but you have to follow the guidelines. Otherwise, they can be great at tracking progress.
Hope that helps.
l2c