I have a pair of the high-dollar ones and I can take my BF three times in a row and it will vary by almost 7%
I also have an electrical impedence scale that does not vary as much as long as you always measure under the same conditions. But the reading is WAY off. Consistant, but off.
I also use mybodycomp.com. They seem to be the most accurate and stable of all of them. It is also extremely useful and allows you to track your changes.
2001 Mustang Bullitt GT #5111 367.6rwhp/378.4rwtq
2004 Black Lightning #789 STOCK: 363.1rwhp/444.8rwtq