Umm, digital pH meters have been around for like decades. They have been used in chemistry labs for as long as chemists can remember. It's just that now they are being made more commercially available (since a lab pH meter costs $500+). Yes, they work fine. However, for a RES, the pH is really irrelevant. You could better invest the money in other items.
As for nitrates, nitrites, and ammonia, all of those can be measured using chemical sensors as well, by even chemists use basic strips for testing things. They are quick, easy, disposable, and cheap. It's like they say, don't fix it if it ain't broke!
On a side note, they do have these like monthly strips that attach to the tank wall that will continuously test those throughout the month. I've seen them in pet catalogs and I think in pet stores.
