Most of us use keywords as they're vital in creating clever and useful smart playlists. I think it's time to get onto Apple en masse to ask for a proper keywords system. Following is my feedback on this matter.
"I've just used keywords in iPhoto 4 for the first time. I didn't use them in iPhoto 2, so I'm not sure if it worked the same way, but I'm impressed with how easily one can add, assign and search for keywords on photos.
"Many people use the Comments field in iTunes to store keywords. This is cumbersome and I would prefer just to use it to store information such as copyright, web page of the author, etc. How about including an iPhoto-style keywords system to iTunes? It would have to be available in smart playlists, eg. Keyword --> is/contains/etc. --> [keyword]."