ClickZ breaks-down new data from Hitwise that suggest, on average, only 85% of brand-name search quearies result in the search landing on the official web site.
I don’t think that’s too much of a shock myself. As the study suggests, among the 15% that stray, a good number click on affiliate links (still the brand). The remainder were probably using the brand-name search as a generic replacement for their search query. For example, someone searching for “Dell” may actually be interested in “Compaq” and “Lenovo” as well, they simply chose “Dell” as the launch point for their “computer” search.