It's only been a few hours, but I'm very disappointed. My first clue should've been that it's listed in the sidebar under "Store".
It pretends to be a tool to share the music you're listening to with your friends. It doesn't. Its goal is to get you to buy music. It's iTunes, of course it does.
On your profile, there is a "Music I Like" section. You can add songs/artists manually, or you can let iTunes populate this list for you. Where does it pull from? Your most listened to music? Nope - your recently *purchased* music. This is not necessarily my favorite music, and iTunes knows this. iTunes even has the data about what I listen to the most already - it keeps track of the "play count" for each song. It knows what music I like, yet it ignores it. There is also a "Genres I Like" section, which it populates automatically. Again - where does it pull from? Yep, purchased music only.
Oh, right - and I can't add an artist to my profile if they're not in iTunes. Which means no Beatles.
Your feed doesn't show music people have played, and it doesnt show music people have rated. It shows music people have purchased, or commented on. And you can only comment on music from the store - not your library. Why would I comment or like a song I dont own yet? And if someone posts a song that I already own, I can still only listen to the :30 preview from that link. WTF?
I'll keep using it, hopefully they'll add more features, and we'll see where it goes. But I thought this was Apple's venture into social networking. It's not. It's just another marketing tool to make you buy music from iTunes.