Hi Nativedc,
Do yourself a favor and go to the advanced search - set the returns for "Most recent topics first" and search on "MS10A" and "Motion Detector", and probably , "False Alarm" (not all at once, of course). Spend an hour of so reading about motion detectors. Rent & watch the movie "Sneakers" if you want a real understanding of motion detectors.
I've found the MS10As to be great, once you have them positioned correctly. You have to understand how they work to get accurate responses and to avoid false alarms. - Not to detail it here, but remember, they "sense" masses of heat moving across their field of view - the greater the differential between the ambient temperature and the moving mass = the greater sensitivity. Watch out for sources of heat, and plan for all 4 seasons. A blast of hot air from your furnace duct at 3:00 a.m. WILL trip the sensor......
And I certainly agree with the other guys here - the object of an X-10 alarm system is to scare the intruders away, a lot of noise and flashing lights is what you're going for and the large PowerHorns make a lot of noise! Take the time to position them strategically.