From what I've seen of iTunes ability to keep album art that I've added or automatically download ones I'm missing, there's no way in hell that I'd let it match my music collection.

Some of the cover art is ok, some is meh (special edition banners), and some is in the WTF range (one zydeco album especially).

Not that I have that much that's not mainstream, but I'm kind of particular about the stuff that's in there.