Very timely, I had been wondering about the same. So I dug one up to check. This one still looks pretty good after one year of operation at one foot (30 cm) depth. Only minimal loss of gypsum at the outer layer. Part of the mesh has come loose, but that has no major effect on the sensor performance.
I monitored resistance before, during, and after removal from the soil (see graph). When removed, it starts to dry out quickly and resistance shoots up. I put it back in the soil and watered to field capacity. The sensor's response and reading appear correct.
Should be good for at least another season.
Continuing live measurement results can be found at this link: