I love most 5 Star products, but this one is an utter failure. AJ has termed it useful as a garden fertilizer, but indicates that it does not do what it is supposed to do.
It is a buffer composed of sodium and phosphate compounds, but it buffers to a pH of about 5.8 in distilled water. So it is useful if you were brewing a beer with high amounts of roast or crystal malt using a DI or RO water. It will keep the mash pH from dropping too low. But for all other cases, it does little. It does not promote a more ideal mash pH in the 5.4 to 5.2 range as the name implies. It does add a lot of sodium to the wort, so there is a strong negative effect on flavor there.
My strong recommendation is that NO brewers should be using this product. The evidence across the homebrewing community is that it does not work and it can have a negative flavor impact. This is the poster child for why learning about brewing chemistry is important to brewing good beer.
For most brewers dealing with alkaline water, acid is an easy thing to add if you know how to do it. A pH meter is a nice tool. But if you have alkalinity information for your water, Bru'n Water has a very accurate acid calculator that was taken from AJ Delange's work and it is proven.
For brewers brewing with RO or DI water, you may not need acid too often. But you will need alkalinity producers like lime or baking soda (chalk doesn't work well). Again, Bru'n Water has the tools to get you right in the ball park with your adjustments.
I enjoyed Gordon's commment in the book where he says, 'don't mess with the water too much'. Since creating RO or DI water is 'messing with' water to the extreme, I'd say that what Gordon needs to revise that statement to is: 'don't add too many salts to water'. In my discussions with Gordon, that is what he really cautions against since he and I have tasted too many beers at competition that have a minerally soda water flavor that doesn't go away.
There are too many water calculators in the brewing world that don't really provide good guidance for mineral and acid additions. Even Palmer's sheet has some outrageous upper limits for some ions that end up letting the brewer 'hang themselves' with too much mineral content in the water. I've tried to make it apparent to brewers using Bru'n Water where the limits are and why they shouldn't go there.
Don't use 5.2 Stabilizer.