I tried out ushare a while back in connection with the media client Geexbox distro. It starts up by scanning all the specified media directories and building some sort of data structure in memory. This is fine for a few dozen CD's worth of music files, but the docs say you could run into problems if your media collection is really big
Another alternative is the attractively named MediaTomb
I don't think I ever got this working. It is a more ambitious project and has quite a few non-trivial dependencies, whereas ushare is dead simple, with only one small library dependency.
I've also tested ushare with a modded Xbox (old model) running XBox Media Center and it works fine (sharing a few dozen files anyway). The XBMC app can also access Samba shares. Can the XBox 360 do that?