I recently did a series of test mashes to put Bru'n Water through its paces. In short, I found it to be quite accurate at predicting mash pH. Details on the test and my results can be found here for anyone interested:
http://restorationbrewing.blogspot.com/2013/03/7-little-mini-mashes.html
http://restorationbrewing.blogspot.com/2013/03/7-little-mini-mashes.html