I know it is controversial but I find it rude, especially when often more than 95% of the plane puts the shades down. If keeping the shades up only affected the person in the window seat then fine but it often affects a lot more. And not everyone brings an eye mask. After all, not all are flying Polaris and not all are experienced flyers.
I like views as much as the next person but would much rather allow my fellow passengers to sleep comfortably.