I used to get my glasses from Zenni, but for the past few years I have not liked their frames. It seems they've gone more to plastic ones, and fewer stainless steel or similar with flexible hinges, etc. Plastic frames are almost always too big for my face, and the ones I do like or think would fit don't work for bifocals. I had no issue with the quality of their work though. I have also tried Goggles4U twice, and had one pair be completely wrong.
The past few years I've been getting my glasses from EyeBuyDirect. Their choices seem fewer, but are actually more varied, so I've always been able to find a good frame. I'm in Canada, so the smaller selection of frames might be due to that. It usually only takes a week to 10 days for my glasses to come, and there are almost always coupons available to cut the price even further.
If my eyes would just stop changing every year I would perhaps be more inclined to buy them from the optometrist, but there is no way I am spending $600 CAD a year on glasses if I don't need to.