Obviously, I disagree with AJ's comments above. I find that very minor 'biasing' of mash pH with respect to either beer style or personal taste does offer improvement in taste and perception. The suggested mash pH range is quite small (5.3 to 5.5), and is well within the range AJ discusses above. I find that basing the pH bias with respect to beer color is helpful and appropriate. I agree that a brewer should base their pH target on what tastes best to their palate and that of their drinkers.
I disagree with AJ on the point about tartness and pH. For a given wort produced with a fixed grain bill, the tartness of the wort will be increased as the pH in reduced. I do defer to the fact that you can't just say that the tartness of ANY wort will have a relationship to its pH. So, he is right and wrong.
The issue with mash pH becomes moot for brewers that brew the same beer over and over again with the same ingredients. The pH meter ends up in the drawer and is never needed after the brewer figures out what their brewing session requires for the same beer. Unfortunately, probably 90 percent of homebrewers and some craftbrewers change their beer style frequently. Those are the brewers that Bru'n Water was crafted for. Giving them a tool with which they can get their mash pH in the ballpark on the first time instead of having to brew and rebrew to figure it out. That is not to say that rebrewing is not an important tool in refining and 'perfecting' a brew to the brewer's taste. For brewers without the benefit of accurate pH measurement, Bru'n Water is a big step up.
Bru'n Water would not be much of an asset for those that brew the same European Lager in large batches. It should be a pretty good tool for brewers that mix it up with every batch. At a minimum, brewers have a tool in Bru'n Water that allows them to calculate what additions in acids, bases, water, and malt will do for their mash pH. It may end up off by a tenth or two, but that is much better than the beer color based programs of the past.