Based on 2 ratings:
The sensor isn’t super sensitive, with the recommended resistors it sits around 1020 in ambient lighting, when you put something close to it it will only change 2-10 digits usually. if light is shining on the sensor it will drop down dramatically (500-900) range. This makes it easy to see if something breaks the light going to the sensor. For our project we had it enclosed in a tube & to sense if there is a ball there or not. So we put an LED across from the sensor so if the ball broke the light it would be obvious to the sensor.