It is as good as the people doing the measuring and the accuracy of the responses they get from the people being measured.
I had great service and accuracy at Lenscrafters in the San Francisco Bay area and terrible results from the Lenscrafters in Sacramento.
Here in Albuquerque, I wanted to see how much damage had been done by the incorrect prescription I got in Sacramento so I went to ophthalmologist who told me I have two astigmatisms, not the one that the Sacramento folks said I have, and that I am at the very beginning stages of developing cataracts in both my eyes.
I still hate my glasses. Even though I have bifocals, I can't see distances well without them and I can't see up close very well with them.
For the most part, I don't wear them at all unless I am driving or am watching a television that is too far away to see without them.