What do I think?
It's too expensive for what I would need (like) from it. Temperature, pH and salinity can be done for much cheaper, oxygen, CO2, ammonia, and potassium are not useful to me as I don't test them, so essentially it turns into a continuous calcium, alkalinity and magnesium checker which while nice, the price of the unit plus replacement discs is too much for me. Plus it simply monitors, if there was some controller functionality to it then it would be quite a bit nicer. You could use a CaRX or 3 part solution as needed to keep the levels you want, rather than trying to make a really educated guess.
But that's just me, I could also put the Apex into a similar category, as currently it would be a really expensive light timer for me as I don't have a need for the other functions it does