Well they are useable, but if you are in a smaller aircraft, you will land long.
Look up the chart and see the eye height for the papi. Look up your flight manual and see your eye height. Work out the difference, then use some basic trig to work out by how much would you land long if you followed it down...
In practice it's not a huge issue, as if the eye height is set for 747 or a380 it will be a long runway. I follow it to minimums then ignore the papi and aim for the markers.