We haven't run any scientific experiments with them, but in our opinion, they probably aren't as accurate as probing the soil with a finger to see how damp or dry it feels. Depending on the size of the pot, feel the soil around one or two inches beneath the surface (perhaps three inches for very large pots), and for most plants, it can become somewhat dry to that depth before the plant will need water. Waterings should be thorough enough to fully drench the soil and have the excess freely drain out of the bottom drainage holes, but infrequent enough that the soil has a chance to dry a bit and "breathe" between waterings. If the root ball is too dense to probe with a finger, then using a wooden skewer, screwdriver, or wooden popsicle stick could work to see if damp soil sticks to the wood or if dry soil is more powdery; the wood may also absorb moisture if the soil is wet enough. (You can leave it out to dry and re-use it multiple times.)

Miri