Well, celebrity endorsers are not all they're cracked up to be. Not only are celebrities fallible--recognizably so, since their lives are constantly scrutinized by the media--but celebrity-driven ads actually don't perform any better than noncelebrity ads do.
According to a 2010 study by Ace Metrix, a company that examines the effectiveness of advertisements, celebrity ads are no better--and in some cases are worse--than noncelebrity ads.
Ace Metrix tested 236 nationally televised ads across 16 industries and 110 brands. The company surveyed between 498 and 608 people per ad (chosen at random to ensure a nationally representative sample), and gave each ad an "Ace Score" based on "Persuasion" (desire, relevance, information, likeability, change, and attention) and "Watchability" (high-, medium-, and low-involvement TV viewing conditions).
The results: Celebrity ads failed to produce higher Ace Scores than noncelebrity ads did. In fact, after the analysts controlled for industry norms, celebrity ads rated nine points lower on average than noncelebrity ads did.
Although Ace Metrix did find some high-scoring celebrity ads, they appeared to be exceptions. For example, the most successful celebrity ads featured Oprah Winfrey, and were styled as public service announcements rather than product endorsements. Ace Metrix concludes that those ads' success cannot be attributed solely to Winfrey's celebrity status, and that the ads might have been successful (because of their messages) whether she had been in them or not.