This is an interesting tid-bit from Jacob Nielsen’s site about boolean search. We know this already, but this is interesting additional research:

Boolean search should be avoided since all experience shows that users cannot use it correctly. We have studied many groups of users who have been given tasks like this:

You have the following pets: - cats - dogs Find information about your pets.

Almost all users will enter the query cats AND dogs. In our studies, they typically do not find anything, since the site never mentions both animals on the same page. Upon encountering a “no hits found” message, the vast majority of users concludes that there is no information available about these pets. Experienced programmers normally use the same erroneous query, but when they get the null result, they typically say “oh, yes, I should have used an OR instead of the AND.”