I use all of them.

Category-this is the base sorting key (like Rock)
--Genre for sub category (like Electronica)
----Sub Genre (like Trance)

This is especially helpful for compilations and soundtracks. You can sort by the sub-genre when the compilation has too many other styles on it.

I really wish there was a category for each track. I use this database to catalog my inventory and to categorize music so I can make mix CD's of my own collection.

It would be really cool to be able to filter out the different styles of some genres/sub-genres to a specific style of a particular track.