The first time a user saw an ad was when he or she was most likely to convert, though the first three impressions all had at least 100 percent lift on average, the study said.
From this article [clickz.com].
AdWords aren't banner ads, but I'm sure some of the same rules apply. I don't know the % of people who do a search for the same KWs multiple times over the course of a week or month before buying products. However, this seems a good case to use multiple ads so that you could be serving a new ad to a user to strike that nerve which gets them to click.
It's also one of the few good arguments for not using ad optimization. If you have a couple ads that outpreform the others, then those ads will be served over and over, instead of letting these type of searches see new ads which might eventually get those more discriminating shoppers.
Any thoughts?
Fresh creative can to some degree counteract list fatigue, but the effect is usually only substantive if the fresh creative has superior performance over the prior control creative. Consequently one should not expect much of a boost from exposing the subjects to lower-performing-but-previously unseen ads.