Just installed it, signed up to the trial. It's got a nice interface for the radio section, but I don't like having my entire music library pushed aside to make way for their subscription service. Thankfully you can turn it off, but I'd rather have my own music categories across the bottom and a little button at the end for this Beats service thing.
It is pretty smart though. I put in a few of my favourite artists and genres and it selected some out-there suggestions that were right (I didn't select electronic music at all but it recommended Kraftwerk, a band I like). Maybe it's pulling data from Genius and our own libraries?
The curated radio playlists are pretty cool, my favourite feature so far. But this is something I'd rather use on my computer not a phone. So hurry up new iTunes app!