I always judge a book by its cover--more specifically, its back cover, where the synopsis usually is. I don't think there's anything wrong with judging a book by its cover design, either. Think about it: if the publishers found a book to be worthy of their money, they would pay some extra cash on the cover. It wouldn't hurt at all for the book to look presentable.
It's not really just about the cover. The first few lines of a book tell you a lot about it. A good author will know how to grab your attention from the very beginning. That's how--I believe--some old novels came to be so well-known as "classics". Printing technology wasn't much, and so it was all up to the skill of the author. It is therefore not so much about the content as it is about the way in which it is presented.
I just plucked a book off the shelf to try it out--one I haven't read yet. It's THE ESCAPE by DAVID BALDACCI. The cover isn't particularly appealing, but I bought it because it was on sale, and I wasn't being the normal me. (I am almost never normal in bookstores.) I think the cover's color is called "turquoise"--some bluish hue. It shows the back of a man running towards a more or less pyramidal door, which also doubles as the A in the word ESCAPE. If you look "less closely" you'll see that you're looking at the man through a breach in a metal fence. I might have read something about the book online, but it doesn't need a particularly sharp intellect to know that's it talks of a man escaping from prison. Not to mention the usual conspiracy, family, and a detective archenemy.
This novel's got a rating of 4.1 on Goodreads, but I wouldn't give it a 3 based on its cover art.
I am inclined to think that whoever said "Never judge a book by its cover" should have put more effort into looking. I know that the writing profession is not generally a superficial one, but looking on the outside is indispensable. Taking the maxim metaphorically, you simply cannot judge a person without first looking at them. We are inherently biased against certain attributes and for others.
Now, I'm not saying that 'beauty' defines how I look at someone, but it is rather their countenance and general bearing. If someone acts haughtily, then I immediately label them as such. The subject matter is too complicated for my linguistic faculty, so I shall stop here.