The music sharing feature is being touted as a killer feature for the zune. I think they've failed in their implementation though. The current design has them copying the songs over to the other zune, and then allowing the zune to play it three times, or for three days, whichever comes first.
The problem with this design is that it highlights how arbitrarily restrictive the drm technology is. When you go back to play the song for a fourth time and you can't, even though it's still there on your device, you realize that you never really owned a zune... at least not all of it. Something deep inside it still belongs to Microsoft.
A better implementation in my opinion:
Allow music to be streamed to any other zune, so long as they are within wifi range. Exactly how iTunes works today. When the zune is out of range, so is the music.
This emulates digitally, what is already possible in real life. If somebody is close to you, and they have music, you can join in listening to it. They could have made it even more interesting by making it so that you can listen to what other's are listening to. Think of the interesting effect that could have connecting other zune owners...
Ah well, instead, it's crippled drm... and I'm not interested.