I've purchased ~6-8 pairs of glasses from zenni, primary because I'm really happy that they don't gouge the heck out of me for lenses with high refraction indexes or various coatings.
That said, the frames are totally hit or miss, with some of them feeling like they aren't worth the $10-20 or whatever they cost. I find though, its no so much materials as attention to detail. The materials themselves withstand a beating. One of my favorite pairs _rattled_ when I got it because the little plastic nose pieces, and the little plastic piece that fit over the end of the arms was lose. Drop of glue in the ear pieces, and a softer nose piece and they are now my favorite glasses.
OTOH, I also owned a half dozen pairs of ray-bans before I switched to zenni, and probably 3/4 of them had finish problems within the first couple years. The coating on the plastic pieces started to disintegrate leaving a sticky residue on one pair, and three pairs with metal frames had the clear varnish like substance start to crack and peel from the metallic finish. Another started to discolor after a couple years. I guess at least rayban has perfected planned obsolescence...