I no longer have a huge iTunes library to run through the upgrade process, but I did do an in-place upgrade with a small library and ran into no problems at all.
Each section gets its own place in the sidebar, which means there's still a lot going on for a music app, but at least it's all music related and no music features have been removed.
That said, it's still the most complex of the three new apps, handling your local music library, a music store (which is still called the 'iTunes Store'), and Apple Music, Apple's streaming music service. It looks like iTunes once did before Apple started bolting all kinds of odds and ends onto it. Apple Music is tightly focused on your music and nothing else.
