I also have a large music library (over 600 folders - one for each artist) and thought I would try UPnP to my PS3. Not only do my media tags not load, but the list took so long to load I gave up. There doesn't seem to be a way to load a playlist that is stored ON the dns, and I can't seem to figure out how to just shuffle music. Unless you have less than 500 MP3s I wouldn't even bother using this box in that capacity. What I've been doing lately is just mapping a drive to a pc and setting WMP or whatever to use \\dataserver\volume_1\documents\music as the library folder.
I still use UPnP for video though, and it mostly does the job.