When shopping, the cover is most important to me because I will never get to the content if the cover doesn't attract my attention. In fact, for me, it is almost imperative. I have been known to buy a book almost entirely based on the cover. Most genre fiction categories have a certain look and feel. When I shop Smashwords, I usually scan down the cover images until one pops out at me as something in my preferred genre that I might like. Only then do I look over at the title, blurb, price, word count. If that looks good then I look at the book detail page for the extended description, tags, and reviews. If there is no cover or it is a crappy cover, or one that is totally off for the genre, then there is a good possibility that I will skip over it -- sometimes intentionally and sometimes because the cover just didn't catch my attention. On a rare occasion something in the title might catch my eye and I'll backtrack, but it is rare. So the cover really starts the ball rolling for me.
Now once I decide to buy the book then the content becomes important. it needs to live up to the promise of the good cover. But ya know, I still want a nice cover in my ereader. The newer Sony shows the covers in the menu and Reader Library shows the covers too. Calibre also shows covers. I hate it when it is a blank cover or shows the first text page as the cover. I like da purty pictures
One thing I have noticed -- traditional authors who start to dip their toe into self-publishing almost without fail have rocking covers! Even for short stories. They get it. They know how important it is. A lot of newbie self-published authors don't. They cut that corner to save money. If they have a knack for graphic arts, then that can work. If they don't, then it can be a big mistake. They will miss the advantage a good cover can give them right out of the gate.
There are books I have passed over because the cover sucked. Then later I saw the same book with a much better cover and I bought it. It was the same content but because the cover didn't speak to me I initially passed it by.
Also, if you give me a choice of two books to promote on the blog and one has a good cover and one has a crappy cover...with all other things being equal, I'll choose the one with the good cover.