Covers make a big differance. Bad cover art turns me away form a book, I don't have a rational reason other than 'instinct.' If I don't feel right about the book, why would I spend $5-$15 on it just to test the theory?
I usually do most of my shopping on Amazon, but that's only for books recomended to me, so cover art in that respect doesn't really matter.
When I do go to my favorite bookstore down in the mall, they don't have a place for me to sit down and try the book. And anyway, I'm a bad judge of whether a book will be good without reading a chapter or two. I am terrible that way, so if my instincts say 'nay' I stay away... Ooh, now I can write a poem on books.
I always judge books by their cover,
I like to say I'm a book lover,
but if my instincts say nay,
I will stay away,
and run like the wind for some cover.
Hrm, it could use some work. I used Cover twice in that poem... What else rhymes with cover? oh well, I just made it in like 2 minutes.
The short answer to your question is yes.