I recently bought 4 pair of glasses* from Zenni Optical (zennioptical.com). I recommend them with some caveats. (If you want to save $5 click my
referral link)
They have a pretty wide frame selection, and lots of options for coatings, tint, lens types, etc. Generally the site works pretty well, you can upload a picture of yourself to "try on" frames, etc. If you have a minor prescription with bifocal or prism, I would say go for it -- you will save $$ and get some cool frames.
I have a very strong prescription, including bifocals. I don't recommend ordering complex prescriptions online from anyone, you are rolling the dice on whether you will be able to see well without eyestrain or headaches. That being said, 1 of the two came out very usable, and the other needed to be corrected (which they covered under warranty).
Customer service-wise they have been excellent. I recently left one of the pairs (my favorite sunglasses) in my wife's van on a very hot day. The coating on them crazed, making them unusable. Zenni asked for a picture of the damage, and replaced them right away, no questions asked.
Overall I would order from them again only for single-vision glasses or perhaps driving sunglasses.
Disclaimer: I used to be an optician so I might have a different understanding/perspective here.
*
Why did a supposedly-mustachian person like myself buy 4 pairs of glasses? Because my forecast of what we'd spend in 2017 in our LPFSA was WAY off, and I needed to use the money or lose it.