From what people on the forum of the Norwegian Homebrewers Organization (Norbrygg) - which I am moderating - say, you really don't need a pH meter if you've got a reliable water report and use Bru'n Water. They report hitting the mash pH with great regularity.
Me, I'm a pH meter addict, though
. And I've brewed enough so that I don't need to calculate the salts and acids any more, so I don't use calculators for that. But I always take measurements of samples during the brewing process, because I've found that getting the mash pH right doesn't always mean you'll end up with the pH you want in the boil, or in the fermenter. And those are important.
So I love my pH meter. I've found, though, that I really don't need to calibrate it very often. If I treat it well, rinsing it with de-ionized water before and after use, keeping it in a storage solution all the time and clean the electrode by immersion in a weak solution of
sulphuric hydrocloric acid from time to time, it remains stable for a long time.