This is something I started thinking about when I purchased my copy of Ariel by Stephen Boyett. One of the things that tipped me over towards buying it was the review quote from S. M. Stirling on the front cover.
How much do those review quotes influence your buying? I know as well as you do that they're there for just that purpose, so I'm wondering how effective they are?
I've been aware of the Diana Gabaldon quotes on some of the Laurell K. Hamiliton novels, but I think I'd already become hooked before I started paying attention to the reviews.
There are a few others that I can remember noting, but can't find now, because I can't remember the book(s) they're on. Generally the quotes are along the lines of the book being "unputdownable" or something like that.
Thing is, you know these quotes are going to be positive, otherwise they wouldn't be on the book cover in the first place. No publisher is going to print negative reviews right on the book.
The case with Ariel is the first one I can think of where the review was the thing that made me decide to get the book for sure, although I will admit I was leaning towards buying it anyway.
In a related note, I'm trying to figure out if these review quotes on covers are a relatively new thing. Laurell K. Hamilton's Anita Blake: Vampire Hunter series has them, Diana Gabaldon has them on the Outlander books, but there aren't any on the front cover of the Camolud Chronicles by Jack Whyte, or the Valdemar series by Mercedes Lackey or the Pern books by Anne McCaffrey. All of these series span at least ten years or more though, so I guess it may be more genre instead of age, although I'll admit that this isn't the most exhaustive sampling to base a survey on.