O.K. - Still very much a work in progress from what Benski's said.
There's no GUI yet so it will only read xspf, not write.
From what I remember, interpreted fields are limited to Location, Creator and Title.
There were a couple of other probs, but can't bring them to mind ATM. (no album field so no way to distinguish different versions, it's first come first served, some quirk with 'The' as well ???).
The Location field is not required if the track is already in the Media Library.
As it stands, I guess the main point behind this was to provide some sort of relocatable playlist.
Such a playlist is content based, rather than location based as with pls, m3u etc. i.e. tracks can be moved around on the disk, and as long as they are re-added to the ML, xspf will find them again, whereas pls, m3u won't.