This year during the superbowl, UCLA neuroscientists mapped brain activity during the superbowl ads, as an attempt to determine which of them was most effective. Their findings?
If a good indicator of a successful ad is activity in brain areas concerned with reward and empathy, two winners seem to be the 'I am going to Disney' ad and the Bud 'office' ad. In contrast, two big floppers seem to be the Bud 'secret fridge' ad and the Aleve ad. What is quite surprising, is the strong disconnect that can be seen between what people say and what their brain activity seem to suggest. In some cases, people singled out ads that elicited very little brain responses in emotional, reward-related, and empathy-related areas.
It should be noted that Iacoboni's maps of brain activity, while suggestive, aren't definitive proof of anything yet. That said, the tests Iacoboni and his colleagues ran seem to support studies that suggest the effects of repeated exposure to ads fall off very quickly:
[I]n some fMRI runs we presented the same ad twice, just to test for habituation. We saw strong habituation effects, such that the second time around the commercial induces much weaker responses.