I tried it a while back, but tagging my thousands of songs was too daunting a chore for me to face, so I gave up on it.
Today, in Google Reader, I noticed Moody again. Remembering it I barely glanced at it but when I did I noticed it said now with auto-tagging. Yea.
Looking at it further, I saw that it also had an online music player that used SeeqPod with the mood-finder.
Look at it. Use it. It's great (especially since it doesn't work very well with Internet Explorer).