I think it all comes down to the care taken in the eye exam. I've tried four different places and Best Optical was, well, the best. Besides figuring out your prescription, they also check for cataracts and glaucoma.
My understanding, and I haven't done it myself, is that the exams are free as what they are actually selling are the glasses. Just go in assuming that is default and see if they will do the exam only for a few.