I've got a fair collection of mp3's most were acquired before ID3 tags were popular and I'm darned if I'm going to redo them all. So I used musicbrainz to retag all my mp3.. Wow - it got most of them but I found one flaw - there doesn't seem to be a standard for the genre's.
One might be: "Rock", or "rock" or even "Rock and Roll" or "Rock & Roll", All of which show up as different genre's on my IPod. I'm a big fan of genre's only because I may be in the mode for rock, dance, classic, or metal depending on my mood.
So - I'm on a quest to find some way to resolve my multi-genre predicament. One note so far, some applications I've found use last.fm to pull tags - this seems like an interesting idea, but may leave me in the same predicament as before.