Important: PDFTK has to be installed, not included! You can compile pdtk with darwinports or download a ppc installer from my site (so keep in mind to install rosetta when you use 10.6)
Usage: You drag&drop you pdf files onto the application's window and edit each files meta data. Beside creator, author and subjec, keywords can easily be added to the files. This is way easier than within Preview or Adobe Acrobat.
Why?: The trick about it: spotlight indexes the keywords stored in each PDF files, so it helps tagging/organzing your PDFs