My homebrew has really improved once I started adjusting my mash pH and building my water. My efficiency went up and it just plain tastes better. Because I started making both of these adjustments at the same time, it is hard to pin point which one made the biggest differene, but my money is on fixing my pH.
I bought this to add a hint of sourness to a stout without having to deal with bugs or a sour mash. I tried to read around an ounce of the acid per 5 gallons of beer seemed like the consensus. Well, that is WAY too much. I ended up with ten gallons of lemonade-sour beer. So, learn from me. For a five gallon batch of beer a quarter ounce will give a hint of sourness. A half ounce will give a noticable sourness (too much for my taste) and anything more than that is a mistake.
I have no experience using this for mash PH adjustment.