I have two pairs of Warby Parker glasses, and I have to give them this: their in-store customer service is very good.
The only downside I have with online glasses is you making adjustments to the fitting of the glasses. Your eye doctor will usually do this for you when you buy glasses for them, but online glasses require you to do it yourself or go into a store (if they have one). With that said, still not a big enough negative for me to go back to buying from my eye doctor.