Clustering versus faceted categories for information exploration

Ok, Google and Google Scholar are not big fan for refining search results by using clustered topics, but Marti Hearst's very informative piece at Communications of the ACM describes the difference between hierarchical faceted categories (HFC) (flamenco, endeca) and clustering (clusty) and their usefullness for information exploration.

Her usability studies shows that "HFC-enabled interfaces are overwhlemingly preffered over the standard keyword-end-results listing interfaces."