I often hunt down specific books I've already been made aware of through reviews, blogs, ads, etc., and at those times, the cover really doesn't matter.
But when just browsing and shopping for a good read, yes, the cover is important. If it's interesting, I'm more apt to pick it up and read the description or click the link for more info. Same goes with the title too to a certain degree.
And I'm one of those who are pretty anal about having the cover display on my reader while in sleep mode
. But pretty much anything is ok for that, as long as it displays correctly, but if there's a few different covers for the book, I will select the better graphic when I add the book in Calibre.