If it's not an author you already love (the way Mercedes Lackey is for me, or J.R.R. Tolkien), I'd bet the cover image plays a part in your choice of a new book to read. They're carefully planned, designed and created to appeal.
That's all well and good, but what about when you're reading/buying a series and the publishers change the cover style completely half way through?
They've done it with Jack Whyte's Camolud Chronicles (although, to be fair, they only really did it after the main series was completed, so theoretically it's quite possible to have a matching set). I'm commenting about this one, because I'm missing one of the Sorcerer pair, and I can never remember which one it is when I'm at the used bookstore.
Diana Gabaldon's books are another example. The first books had the strip with the title in the bottom third of the page, and a painting/mixed image for the rest of the cover. I'll admit that that was one of the things that attracted me to the first book, Outlander. Now, it's a plain solid cover color with a simple celtic ornament in the center, and has been from The Fiery Cross onwards.
Sherrilyn Kenyon's Dark Hunter novels are another example of reprinting with new covers. This one downright confuses me some of the time, as I have them in one set, and the ones in the store are different. At least it's not too different in style (up until Acheron and Bad Moon Rising, anyway). However, I still think of Night Play, for example as being the pink book. now the main cover color is blue. Kind of throws me off a bit when I'm searching them out for customers.
The other headache in covers/editions for me is the History of Middle-Earth series. There's so many editions, and all have different page numbering.
The Pern series by Anne McCaffrey are another set of books where the covers have changed over the years. Some of the earlier books have been reissued in the new covers, but not all, I don't think. Also, the Anita Blake: Vampire Hunter novels by Laurell K. Hamilton are mixed like this. The change is at Obsidian Butterfly, I think.
Question is, are you collector enough to want to have matching sets for books? If so, do you do anything about the re-issues (such as re-buying the series), or do you just gripe mentally, and put up with the multiple cover styles?
Personally, I tend to put up with it (although, as I said, I'm hunting certain exceptions in the used book store). The one exception is the History of Middle-Earth series. I'm trying for the black HarperCollins trade paperbacks, simply so they'll all work with the index volume, which I intend to buy.