When Oprah first started her book club many years ago, I paid closer attention to her recommendations. Now, I look more for books that interest me, regardless of who recommends them or if they are even recommended at all. I am more likely to read a book if another reader I respect has read the book or reviewed it positively. I also have more time to investigate better sources for books than I did when I was working full time.
I agree with many of the posters that endorsements draw attention to books that many might not pick up otherwise. And I am glad that some celebrities focus on books rather than make-up, clothes, or purses. :)