You might be interested to know that I’ve developed a technique that can be used to work round some of the limitations discussed in this thread – top 10 lists, etc. It involves creating a static playlist instead, via an automated process.
See here for more details: http://www.andrewferrier.com/blog/2008/01/05/more-flexible-firefly-smart-playlists-with-perl-sqlite3-and-m3u/