Well it depends on how and where you measure. Most of the times their only directions is to measure at your waist, which if you measure there, and you have fat in other places or you gain fat in other places more, they are not accurate at all. And the same is true if you mainly gain fat at just the waist, and not the other body parts.
Now if they give you the chart to measure arm fat, waist fat, hip fat, thigh fat, calve fat, back fat. They they are a pretty good set, if it's just measure at your waist and see the CMs of fat. They aren't very good. But you still can use them, as you can at least measure your waist and watch the fat go bye bye from there. You might not havea good percentage, but you at least have method of documenting where you started with the CMs of fat and where you ended up. I know it's probably not excatly what you wanted. You probably wanted a nice solid BMI reading that you could base things on. What I tell all my clients who like to watch the BMI readings, is to use every reading you can that measures differently add them up and then divide them and then you'll have a pretty close BMI, to what the health gyms would get.
Also it's kind of fun to see what body part is actually more fat then the others, it can suprise you sometimes. As you may not have thought that area was that big and then you find out its mostly fat.
Good luck
Kit