With the passing of three very different celebrities this week (McMahon, Fawcett and Michael Jackson), USA Today mentions Madison Avenue will lose three advertising spokespeople - http://tinyurl.com/mpfuuc.
I've always been a little leary of using celebrities to endorse products. I guess I've seen too many do something unfortunate and get caught in a negative publicity sprial. The spin then gets attached to the products they're endorsing.
The risk of this versus the attention your ad gets from the celebrity should always be weighed. You also need to evaluate your objectives in using a spokesperson. Is it because you're a fan or because your target audience will pay attention? And will they remember your product? Because that is, after all, the purpose.
An obvious consideration is credibility with your prospects - would this person use this product? This rule, however, can be broken successfully. Think Joe Namath in pantyhose. But I don't remember the brand. And wasn't that the point of the ad?