PIR sensors are cheap and easy to use but they suffer from slow response times and low repeat rates.
A PIR sensor works by detecting the thermal radiation given off by a warm person as they move across the field of view. Using a fresnel lens they convert the motion into a changing level at the sensor which triggers it.
Often they have a switch inside for 1 or 3 pulses before they trigger. I always move this to 1 pulse so that I get a faster signal and greater sensitivity to movement. The downside to doing that is false triggers but I handle that with the probabilistic treatement I apply to all sensors.
Battery-powered PIR sensors for alarm panels may have repeat rates as low as once every three minutes. This is fine for an alarm panel but somewhat useless for home automation, avoid them if you can. They are however worth adding in places where you otherwise can't add a sensor because there is no power source or wiring.
I've been working on home automation for over 15 years and I'm close to achieving my goal which is a house that understands where everyone is at all times, can predict where you are going next and can control lighting, heating and other systems without you having to do or say anything. That's a true "smart home".
An overview of the many sensors I've experimented with for home automation including my favorite under-floor strain gauge, through all the usual PIR, beam and contact sensors to some more esoteric devices like an 8x8 thermal camera.
Bluetooth sensing for home automation is a great proxy for people counting as it can detect and locate each cellphone in the house. iBeacons attached to tools, cars and pets can provide a 'find my anything' feature too.
Having at least one light sensor is critical for any home automation system that controls lightng. Lights need to be turned on when it's dark not at specific times of day, especially here in Seattle when it can be dark and cloudy at any time of day.
Microwave doppler sensors can be found in some alarm sensors but there are also available very cheaply as a separate component. They offer exceptional range but suffer from false triggers requiring a probailistic approach to people sensing.
An if-this-then-that style rules machine is insufficient for lighting control. This state machine accomplishes 90% of the correct behavior for a light that is controlled automatically and manually in a home automation system.