Whether a pH meter is critical to your brewing success depends on your water and the kinds of beer you brew. When I first started brewing all-grain my efficiency was terrible (40-45%) and my beer was often a bit tannic. It turns out that I have water with pH 9.3-9.6, and I wasn't correcting for that. Beers with a lot of dark grains could get the pH down to the good range, but others couldn't. Now I use Bru'n Water to guide me in adjusting the water chemistry and getting the correct mash pH. It is doing such a good job that the measurement is almost a formality now, and my beer tastes a lot better.
--GF