The really excellent homebrew site brulosophy focuses on doing triangle tests to determine which factors have the strongest influence on the resultant beers - while a shocking number of things they've tested don't reliably produce a noticeable change in the finished product, water chemistry was one of the few where there was a reliably significant difference.

http://brulosophy.com/2015/04/06/water-chemistry-pt-1-mash-m...