I think that celebrity advertisements can be appropriate, but in most cases should be avoided. For example, if a well known NHL Hockey player like Alex Ovechkin was to advertise a CCM brand hockey stick that he actually plays with, the advertisement would be honest and effective. However, if there is an advertisement that uses a celebrity that clearly isn't using the product it makes me question the honesty and integrity of the company. An example of this is Justin Bieber (new young artist) who advertises for Pro Active. Not only is Bieber about the 5th celebrity that Pro Active has signed to do their commercials, but it is highly unlikely that with a celebrity income, they are using Pro Active to wash their face. Pro Active is targeted at middle class families who need a product to help them clear up acne. Celebrities are much more likely to use more expensive products.
I agree with Laurie when it comes to potentially taking a hit on your profits when celebrities get a bad reputation. However, I don't think this would necessarily devastate any company that is able to afford a celebrity endorsement in the first place. All in all, I think it's great if celebrities want to advertise products that they actually use, but I don't like to see celebrities advertising products that they do not use.