I monitor pH, pH in my ca reactor, temperature, amp draw and salinity. I also watch 6 different float switches. 3 in sump, 3 in ATO reservoir.
Honestly? They aren't super accurate. Proper calibration makes them really accurate at the time they are calibrated... but they seem to drift over time. What they are good for is noticing large swings. So, in Kevin's example, it would work really well. I tie salinity into my ATO code. If the salinity is lower than 33ppt, it won't ATO and will alarm/email me. Works really well. The only time it's had to cut it off, it was just a dirty probe from an excessive phyto feed. Shook it off and all better.
Jesse, let me know if you want to do the Salinity probe thing and I can help you out with code.