I am a Doctor of Optometry. I have worked in ophthalmology offices, mall/retail offices and am currently in private practice. Here are some general observations on the different types of offices:
1) Ophthalmologist specialize in surgery and disease treatment. In many of the offices I have been in, the ophthalmologist will spend very little time with routine vision patients. In many of these offices a technician who has been hired and trained on the job is doing most of the exam including the measurements for the glasses prescription called a refraction. Optometrist spend four years learning these skills and I believe you will get the best refraction and glasses prescription if you visit an office where the optometrist or ophthalmologist personally does your refraction.
2) Many of the retail type optometry offices who advertise low priced exams do not include the cost of a dilation in the exam. Since ophthalmologist concentrate on eye health issues, they will include dilation in almost all of their exams. If you go to a doctor who adverises low prices ask if they charge extra for the dilation or other necessary test. This is an important part of a complete health check.
3)Beware of low priced ads for glasses. Particularly in offices that run low priced glasses advertising, the sales people are trained heavily in "sales". I have known patients to be told that they needed such and such option added to their glasses when indeed this was not true. Also as the previous poster said the low priced items offered are many times not the most appealing styles. The ads get you in the door and the salesman may try to give you many extras. Some extras are beneficial and valid reccomendations, but like the previous poster said, your best bet is to develop a relationship with a doctor and ask his/her recommendations.
4)Remember, you do not have to buy glasses at the time of your exam. If you want to shop around take your prescription and compare what you are told by different glasses salesman.
I hope this gives you some insight from someone "on the inside". Please don't assume that the problems above happen everywhere. I just want people to be aware of what to look for.