With all due respect, I strongly disagree.  Those small cheap ones are very inaccurate,  and you really need to spend closer to $250 used or $850 new.  This one is pretty good; my doctor has a professional version of this, which is about double the cost:
Tanita TBF-300A body comp. Scale - eBay (item 300418440775 end time Apr-23-10 10:06:03 PDT)
However, the only true measurements are the "bath tub" type, where you are dipped in water.  That's obvioulsy only for a competitive pro. 
Charles