Just about every book has at least a couple: the cover quotes, usually from some well-known author who writes in a similar genre. The question is, how well do they work for you? Do you pick up a book and go "Jim Butcher and Patricia Briggs liked this book, THEREFORE IT MUST BE AWESOME!"? Or do you tend to ignore these things completely?
I'll admit to having a fangirl moment once or twice when it comes to cover quotes, but most of the time I just assume the quotes are a collection of PR-worthy phrases from the author's friends. They don't often tell me what I want to know. "An edgy, dark fantasy" says nothing more than I worked out for myself from checking out the cover art and back blurb. And let's face it, although every single sword-and-sorcery fantasy novel ever has been compared to Lord of the Rings at some point, it rarely has more in common than genre.
That being said, there are occasionally cover quotes entertaining enough to make me want to pick the book up in the hopes the actual story will provide as much amusement. (Check out
Sherrilyn Kenyon's quote on Kinley MacGregor's book... and then take note that they are, in fact, the same person writing under two different names.)
So. How do cover quotes affect you? Can they be an effective marketing tool, or do they just take up space on the cover?