I've been sticking with iTunes 6 because of its ability to convert my downloads to WMV format directly, that is, without going through the "export to CD then reimport to iTunes" workaround. (I use only WMV format in my slide shows.)
But iTunes 9 also has that capability PLUS all the following:
--Home Share: easily share to other computers, over your home network, all your downloads
--Genius Mixer: quickly find songs that are similar to one another and that iTunes will group together for playback
--newly-designed iTunes store: vastly more convenient, and faster, browsing of the store's contents
--"iTunes LP" and "iTunes Extras"--download a selection and also receive extra content (lyrics, images, posters and lots of other stuff) that are exclusive to iTunes