depends on the caliper, some make it easy to read the results, while cheaper ones make it harder to read accurate results, and then it also depends on the test you run with it, you have multiple areas on your body to perform the test, and the majority of these tests require another person to measure them on you, so i guess it's accurate if you get a good caliper and do it on yourself or get someone else to do an extensive test on you
check out this website to see what tests you can do, where on your body to do it, and calculate your body fat
www.linear-software.com/online.html
the "Jackson/Pollock 3 Caliper Method" is the easiest to perform on yourself