Would it be possible to use other sources? Two possibilities:
-wikipedia: more correct information but not that much info, certainly not for individual tracks (and even albums)
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www.allmusic.com: very qualitative source. They have one genre and styles for every artist/album. Genre would be eg. electronic (they never use more then one genre) style would be eg. "Big-Beat; Electrofunk". Styles are very correct. (Even for more exotic music: styles like Cajun; Zydeco and Zouglou are used properly.)
Only "problem" is that they use Pop/Rock as one big genre. (So Slayer and Brittney Spears have the same genre.) But it's very easy to split that into Pop or Rock manually...(Or just delete the crappy pop music

) I think i will use the "genrefield" in mediamonkey for all the styles (rarely >3) and put the main genre (never >1) in a custom-field.
Other big advantage: the use of moods, instruments, country, years active, themes, label, release date, record date, group members, influenced by, similar artists, albumart and some nice comments. For artists, albums and tracks.
Eg. for the use of genre / style / mood:
Arcade fire - Neon bible
genre: Pop/rock
styles: Post-Rock / Indie Rock
mood: Dramatic, Earnest,....
Theme: Road Trip, Introspection,...
Last.fm might know a little more artists, but on the other hand, "smaller" artists are never tagged correctly on lastfm.