I have bought glasses from Zenni and they were fine, although they really really don't like to make bigger lenses (the last time I asked for frames with 40mm lenses I got exactly 3 choices). I expect that this has something to do with the lens making equipment they use.

I'm looking forward to the flat optics future when a holographic lens gives you the same correction but without the weight, depth, or chromatic abberation of a "normal" lens.