For me its easier to use a timer. That would probably work just fine but it would have to be tuned for the best results, just like a timer does. The one good thing about it though is as your plants' water needs change, the sensor will take care of adjusting for it, rather than having to reset the timer.
The weak point there however, is the sensor will go in 1 plant pot but if the system serves multiple plants then you have to be sure that they are all drying out at the same rate, or move the sensor to an "average use" plant, and then adjust it so that the plant that uses the most water (ie. because it is closest to a fan and dries quicker, or is a strain that drinks more) doesn't get too dry between waterings.
