In our experience these ratings are very optimistic… In tests, we have had an 18lb dog set off these motion detectors. While they only tripped the sensor about 10% of the time, that is still unacceptable for security.
Another problem we have seen is in homes with 2 or more dogs. Two 20lb dogs chasing each other look like a 40lb + dog to the motion detector.
We still love motion detectors, especially for their ability to trigger lights and music as you move throughout your home. However, if you do have dogs we recommend also securing your doors, windows, and in same cases implement glass break sensors.