Sigyn, you're in my writing group and you know I love your writing. I think it's safe to say I'll cover quote you...
Cover quoting is a strange business. It's left completely up to the author. A publisher will send you a book, sometimes just out of the blue, and just ask your opinion on it. Sometimes, another author will ask you if you'll look at one of their books. In the end, the decision whether or not to quote (or what to say) is up to you.
There are a lot of factors in it, however. Sometimes, those asking for quotes will be your friends. Sometimes, they'll be important people in the field who have helped you out. Sometimes, you'll read a book and really like it--but not be able to decide if your fans would like it.
Also, there's the time factor. Sometimes, I get a book to read which looks really good, but I'm too busy to read anything at all. Sometimes, I'll get a book that isn't quite as brilliant, but I need something to read.
And, once you've read and enjoyed an entire book (even if you didn't love it), you are left wondering what to do. You just read the whole thing. I feels like a shame not to offer something at that point. Plus, new writers need every break they can get. It might not be ENDER'S GAME, but if the author did a good job and shows potential, you might quote it anyway.
I always try to quote honestly. I don't gush if I didn't love the book. Instead, I point out what the book is like and what its positive factors are. That way (my theory goes) if my readers pick it up, they can know what--exactly--I liked about it, then have more information to judge whether or not they will like it.