Yes. They are crap. I have tried several of these devices (luckily never bought one).
Apparently, muscle and fat give different readings back from the electrical current sent out. Unfortunately, this is highly effected by your body's water content, as well.
So, if you are dehydrated or really hydrated, it will be a HUGE difference.
Basically, they are the worst devices in terms of accuracy.
The best way to get accurate reading is the water-submersion method. But you would have to find a facility (medical center maybe?) near you. I assume they would charge you for this. No idea how much. Even this is only accurate to +/- 5% no matter what anyone tells you. That's because air in your body causes more water displacement, and you cannot expel all the air out of your lungs - let alone all the air in your gastral-intestinal system.
The next best method is the caliper method. If you get a good device and an experienced bodybuilder trainer at the gym that takes readings all the time - you'll probably get a pretty good reading that will give you a good idea of your BF. Just make sure to get the same person to do it to you every time - make sure they test the same spots - and try to do it before you work out. I have read that post-work out, readings are more inaccurate due to tissue inflamation.
Whew! That was a book.
Hope that helped.
-j69
