It works fine. Note that input voltage MUST be in range 3.6 to 6.0V (maximum rating of used LDO regulator). Output analog voltage is from about 2.83V down to 1.73V, higher moisture = lower voltage. Output voltage of course depends on penetration depth, deeper = better sensitivity, so put it into the soil up to the line drawn on PCB. Design is simple, TLC555 generates square wave signal, which goes into the low pass filter compund by 10k resistor and "sensor capacitor" (created on PCB) - higher capacitance, better filtering, lower amplitude. This signal is than captured through diode at output capacitor (peak detector with slow discharge).
Current consumption at 5V is about 4mA.
If you want to use it at 3.3V, you can use it as is just note that LDO will be in dropout state so it will not filter the input power and output voltage will be lower than show in attached picture. Or, you can remove the LDO and short it's IN and OUT pads on PCB by wire.
I have drawn the schematic and made some measurements for different conditions: put on free air, in the dry and wet soil and in water (100% and 50% in it). See the attached picture for results.

They look very nice, haven't yet used them though.