currently I've organized all my media by setting the custom field 1 (named "Category") to "Popular", "Classical" or "Audio Book" (in MM4 I will surely add the category "Video").
I use filters to search, browse, play from one of these "sub libraries" (BTW: the possibilities of these "filters" in the latest MM 3 are already extraordinary!
A few features I still would like to make this easier would be:
a) the possibility of displaying a ListBox above the tree view to select one of the defined filters (maybe optically similar to the media type selections in iTunes).
b) an index i.e. auto fill function for the custom fields (so that pressing the first key (in my case "P"/"C"/"A"/"V") is enough to set it's value).
c) ripping a cd, when a filter is active presets some tags with fixed values (e.g. Filter "Audio Book" active, ripping a CD sets custom field 1 to "Audio Book")