I buy most of my books from the used bookstore, through Amazon (essentially sight unseen), or as ebooks, and I don't consider myself terribly hung up on the appearance of a book- with the exception of movie tie-ins, which are always terrible. In all honesty, I can't really afford to be terribly picky, and so own some pretty egregious examples of bad covers. I have entire series where each volume seems to be attempting to one-up the previous entries in a contest for ugliest cover art.
And some of Stephen King's old covers? Blergh.
And some-probably most- that are serviceable, but boring.
So it's really hard to single out something bought "despite" the cover, since it seems that a good chunk of the books I own were purchased without much regard for appearances. Maybe the worst offender so far has been this edition of Buddenbrooks, by Thomas Mann. Not only is the design dated and uninspired, the cover itself is rather inflexible and makes reading the book more difficult than it should be.
In honor of today's #bookadayuk topic, here is a brief list of some really terrible book cover redesigns that have made an appearance on shelves just recently: Truly Terrible Book Jacket Redesigns (Flavorwire.com)