They work great. However - remember the movie "Sneakers"? Motion sensors operate on a temperature differential. Ambient vs. an object. The object must be warmer than ambient - and the more the difference is, the more sensitive the sensor. The object can be a "cloud" of hot air. Don't position the sensor where it will "see" the blast of hot air coming from your heating ducts, and putting it directly above your home theater setup might result in your neighbor calling you at 3:14 am, Christmas morning - being just a tad ticked-off at one of your false alarms.....
And - set the switch to "2" hits to make a trip.
If they pick up critters on the ground, add some masking tape, ½" at a time, to the bottom of the lens.
Most important - search & read here.