I mainly buy cheap sunglasses for the main reason that I always break them (usually end up sitting on them) I have though got some mid price nike sports sunglasses that I'll use this summer.
I wonder if the designer sunglasses seem to last longer because we take a lot more care of them.
I know if I'd paid £50 + on sunglasses I wouldn't just throw them on the lounger/sand etc like a cheap pair!
My wife thinks I'm a bit mad cos I take about 5 pairs away with us but usually bring only one or two back as I break them!
On a side note, I tend to like the very dark or mirrored ones as you don't get caught checking out the ladies so much!!!