I have the SmarterSpa and am really, really happy with it. Their sensor is just measuring voltage across the sensors, I think. You can adjust the level at which it starts a generation cycle, and you can adjust the length of the generation cycle. When I asked them about the sensor they were kind of vague about how closely this correlates to chlorine levels, but in practice, two months in, it is working like a champ. I am down to testing my levels every couple of weeks because it's just that stable. FYI I use the tub most days, and some days twice. It's usually just me but I am occasionally joined by one or two more people. If I had a heavier load I imagine I might have to add bleach sometimes but that's just conjecture on my part at this point.
Let me know if you want details on what I've got it set to - I dropped the start point by a couple of notches, I forget what I have the run time set to.