I wore $10 sunglasses all my life. Got tired of them breaking all the time (I used to carry them around in my cargo pant pocket when not using them), I got an expensive set of Oakley's. They're expensive enough that I am forced to take good care of them, use a nice case etc. Had I taken care of the cheap ones, I guess they'd last just as much. These Oakleys are light and comfortable, for sure, but probably not worth the money for me.
As for looking pretentious, I don't care. I'm in SoCal so they help with the sun, and in addition I commute by a fast bike and they help vs the wind. They are not a fashion statement for me.
But I am slightly put off by people who wear sunglasses that are more for fashion then utility, like the Blues Brothers Ray Bans and those woman's cover-your-face lens sizes. People wear those all the time in my beach town. I'm also a recent immigrant from Europe so there you go.