Unique to the Vita is a touch interface, allowing for easy navigation and playlist management. You can also download and store songs for offline play, which is a brand new feature for the service.
I'm dicking around with it now. It's not bad, although my Vita went into sleep mode during a song, messed up the connection, and now it won't reconnect at all, even after closing the application entirely. There's no option to reconnect manually and refreshing it does nothing. That's not exactly convenient.
[Update: If Music Unlimited loses its connection, you can re-establish the connection by opening a different application that connects to the Internet, like the PS Store, and refreshing the connection there. Then you restart Music Unlimited and it'll reconnect. So ... that's ... elegant.]
Still, the apps themselves have all been surprisingly good on the Vita. Netflix works well and I love the Twitter software. If I can get this bloody thing working, I might actually like it.