I've used the product for several years and am on my second tub of it. I started using it when brewing in Collingwood and just continued using it when I moved to where I am *right this very instant*. After hearing Thirsty Boy's arguments and seeing his results, I thought I would run a few brews through my system without it.
I brew with rainwater collected on a ColourBond roof into a galvanised iron tank, pumped up a hill through black poly pipe into a concrete storage tank, which then falls back down another piece of black poly pipe into my brewery. There's a good mix of eucalypt, dust and birdshit in there which either gets dissolved or falls to the bottom of one of the tanks. I don't currently filter at all and, if memory serves, brew in an aluminum pot.
I've never measured the pH of any water I use - I used to tell myself that buying this product would fix any problems so I don't have to measure the solution before/during/after. I remain happily ignorant of what is actually going on. I use it at a rate of about 3grams per 24litre finished wort; perhaps a little more for big or particularly dark bears... I say this because I measure the water into the mash tun in centimetres and not litres.
What I can say is that for the half-dozen brews where I did not use the product, I found I had (only) slightly lower extraction efficiency, but I was quite unhappy with the general outcome of my beers. I thought they tasted different - perhaps slightly astringent, certainly the flavours I was chasing were less well-defined - and they appeared cloudier than I recalled previous brews.
Since returning to using it, I believe those issues have gone away. I have also made other improvements in my process, but there was enough for the pattern-matching part of my brain to register a correlation.
Note that my description and explanation are completely unscientific. There may be a confusion of cause and effect. But, it's all good enough for me. If it turns out that the good work done by knowledgeable and respected people differs from mine, then it won't be the first time. </Normal_Waffly_Disclaimer>