* Users need to filter content using multiple taxonomy terms at the same time.
* Users want to combine text searches, taxonomy term filtering, and other search criteria.
* Users don't know precisely what they can find on your site, or what to search for.
* You want to hint users at related content they might not have thought of looking for, but that could be of interest to them.
* You want to clearly show users what subject areas are the most comprehensive on your site.
* You are trying to discover relationships or trends between contents.
* Your site has too much content for it to be displayed through fixed navigational structures, but you still want it to be navigable.
* You want to use a faceted classification because a single taxonomic order or a single folksonomy is not suitable or sufficient for your content.
* Users often get empty result sets when searching your site.
* You think that "advanced" search forms are not fun to use.