i have been using it on my system and it has worked very well, i have checked it with both refractometer and a digital refractometer and it stays real close if not right on. it did take me a bit to dial it in and you have to make sure there are no bubbles in it and my probe is in the sump with the skimmer so if the skimmer goes nuts it will get bubbles in the probe and mess it up.
other than that it works very well, the only probe i am not sure of and need to evaluate is the orp probe...