My leather cover came from Brookstone. It was a gift so I had no say so in it's purchase, but it has turned out to be the best cover. After two years of constant use, it still looks like brand new. Stands up well with it's build in stand. My favorite part of it is it's rough texture. It clings to the arm of the chair and even stays there if I get up from the chair. The rough texture allows it to stand up against a pillow without slipping, so I can use it lying down in bed.
I have posted elsewhere on this, but I really like the BookBook leather cover from the Twelve South company. They make really great products, stylish, well crafted and durable. I have one of these covers for my iPad mini (though they make them for the normal iPads as well), and another for my MBA.