Sensitivity to low PH varies, but just about everyone will be complaining about burning eyes when the PH is as low as 6.8.
The real problem with a PH reading of 6.8 is that the most common PH tests will report anything lower than 6.8 as 6.8, so there is no telling how low the PH really is. Really low PH can seriously damage the pool in various ways. It is best not to take that risk.
It isn't all that common, but adding pool chemicals in the wrong order and cause calcium clouding, which can't be filtered out and often won't go away until you adjust the chemistry correctly.
I'm not really sure what kind of "chlorinator" you are talking about when you say "chlorinator pays for itself within a few months; after that you're saving heaps of money". Assuming you means salt water chlorine generators (SWG), they tend to break even with other forms of chlorine over the lifetime of the unit. With a SWG you spend the money up front, rather than slowly over time. The final cost tradeoffs can come out either in favor or against depending on what prices are in your area.
