It's been five months now since I got a pair of Ray-Ban Aviator sunglasses and I can safely say that they're no better in terms of functionality than a $10 pair of cheap sunglasses when those cheap ones are new. They don't fit better, they don't let me see better, they don't even really look better on me.
However, six-month-old cheap sunglasses would be scratched by now, and impossible to really get clean of smudges. The Ray-Bans still come perfectly clean every day, have no scratches, and work as good as they did on the first day.
Thing is, they still cost ten times as much as the cheap sunglasses. So if they only last five times as long, they're clearly not worth it. If they get close to ten times, they might be, because avoiding the hassle of dealing with replacing cheap sunglasses more often would tip the balance. Plus it's not like the cheap ones go bad all at once; they gradually degrade, so any measurement of "how long they last" has to account for the fact that there's a certain amount of that time when they're not working as well as you'd like.
But even accounting for that, Ray-Bans will have to last a lot longer to be worth it. So far these are doing fine, but it would be so easy to drop them, lose them, etc. and you have to also account for worrying about that in the cost-benefit analysis too.
So when these are gone, I'm going to go back to cheap sunglasses, most likely.