Zenni will give you glasses that are functional, I suppose, but they do cut corners. I've tried a handful of the online glasses shops and have never been terribly happy. The frames are cheaply constructed and not terribly durable and the lenses seem to be of poorer quality. If I needed a spare pair to take on a canoeing trip I'd get them from Zenni or some other cheap place so I wouldn't have to worry about losing an expensive pair.

In terms of comfort and vision I'll pick the expensive glasses every time.