The Short Version: The Insteon motion sensors cost nearly ten times more than the X10 motion sensors but they are very reliable (as long as you have the necessary APs to send the signals over a reasonable distance or through several walls).
Ever since I started using home automation devices I’ve relied on X10 motion sensors for several different tasks. Unfortunately, I’ve noticed consistent communication problems between the X10 devices and the computer. Due to the manner in which Insteon devices communicate I haven’t noticed issues with those devices. Some of the X10 light switches don’t respond from time-to-time. That’s not as big of a concern as having problems with the motion sensors.
Reluctantly (because of the cost), I ordered Insteon motion sensors to replace the X10 sensors. I paired the sensors with the PLC and then added them to the device information in mControl. Communication problems seemed non-existent. Sure, they cost considerably more than X10 sensors, but what good are X10 sensors if they don’t work as needed?
The Insteon motion sensors also have a red LED that pulses at a regular interval. However, unlike as with the X10 sensors, it’s very noticeable. Fortunately, it can be disabled, which I did with one of the sensors.
Updated 12/12/2011: Don’t forget to link your motion sensors to an AP or the controller itself.