So far it's working exactly as expected for me, although I expect that's in part because I was already using Match, and I was already a Beats subscriber. There are still some quirks in how everything is integrated, but so far so good. I will say that I have not yet updated my home iMac which houses my main iTunes library as I've been away from the house this week, and yes, I would be pissed if it messed up that library that I have been carefully managing for more than a decade, but given my experience so far across a couple of iPhones, Macs, and iPads, it looks to be operating exactly as I'd expect.
It will be interesting to get my wife's feedback as she starts to use it though. She has been mainly a Spotify user for the last couple of years (we share a subscription with her being the primary user). Still, Spotify has had plenty of it's own UI oddities to get used to, and I have to say that her playlists are a complete mess, and often finding things in the various UI's is no picnic. Still, she is often a reality check for me with these things as she represents a much more average user.