Overriding factor, no. Will certain covers attract my attention while browsing? Certainly. It will influence my purchasing (or borrowing) a book since I clearly cannot buy a book that I'm not aware of. Will covers detract from buying a book? In some cases. If it portrays images contrary to my personal values or reading tastes, I'll bypass it. If it looks unprofessional, I'll probably bypass it too. (If an author/publisher doesn't care about a professional cover, why should I believe that they care about the quality of writing or story telling?)

I will add that I find anyone who claims that they aren't influenced by the cover at all is a bit suspect, unless they know which books they are going to read before they go shopping. There are just far too many books out there to avoid consciously or subconsciously filtering by the general appearance on the shelf.