Hi Wattlebloke,
I haven't tried one yet, one of our suppliers at work sells them and has been trying to get us to test them out in the field, accuracy is apparently good, I assume the better quality probe you buy the more stable your results will be.
If you want to buy one, I can pass on an Australian suppliers contact details in a PM. I can also point you in the right direction for probe suppliers.
The only reason I haven't brought myself one is I use my work pH meter with another pH probe I use for cheese.
Mathew