Bought a set of these for my wife’s office plants. So far they work exactly as advertised and the plants are healthy and happy. I used one in a six inch pot and two in a 12 inch and that seems adequate.
For folks that are having trouble with over watering, make sure that the water jar is BELOW the bottom tip of the watering stake. (which is mentioned in the instructions but the illustration is vague) If it is level or above the water stake will draw too much water.