I've had great experience with Zenni for single-vision prescriptions, but when I graduated to progressive (i.e. bifocal) lenses, the Zenni lenses were terrible and I couldn't use them. Not sure if it was the frame fit, or poor PD measuring on my part, but I tried twice and both times I couldn't use them.

The glasses I got from my optometrist are much better, at 3X - 4X the price of Zenni.