I was skeptical initially. I entered my full prescription data into the site, selected the lenses, asked their customer service people for advice, packaged up my glasses and sent them. To this point, I am doing exactly what your optometrist does, except I wasn't paying them as a middle man with a retail storefront.
If I sound exasperated, it is because I wandered into an eyewear store last month for a checkup. On the way out I stopped to get my first replacement pair. Cheap place. I thought, why not. 30 minutes later, I knew too well why not. My saleslady didn't understand lens materials, thicknesses, tint vs. polarization. Nothing. Just "These are thinner." Really? How much thinner for my prescription (modest)? "Dunno, but they are better." Ugh.
I went to Costco a week later and got better service and ended up with glasses at half the price. I am done with retail eyewear. So when it came to replacing lenses (Costco won't do these two), I needed a better option. Eyeglasses.com was recommended (Thanks Matt), and I am not looking back.