It works well, except in dark areas. What it does is if your timeout is set to 15 seconds, right before the screen times out the phone will do a 1-second scan with the front facing camera to see if it can detect a face (ie someone looking at the screen). If a face is detected then the screen stays on for 15 more seconds, and another scan is performed. If you set your timeout to 30 seconds then the scan is every 30 seconds and so forth.
It works really well and I've noticed I can't use the phone without it if I'm reading long articles since it automatically prevents the screen dimming when I'm reading. The only exception is nighttime reading when the scan has trouble detecting faces in dark areas.