I've received my HomePod(s) now and begun to listen to music - I can say conclusively that they play anything I want to select from my own personal iTunes library. The HomePods were immediately available as AirPlay speaker choices on the iTunes menu, and I can select one or more speakers (HomePod, computer, other, in any combination one or more) to play my music.
For me, this is exactly what i was hoping for - I can now put speakers throughout my home and direct my iTunes library music to them.
The Siri control is not as complete as it will be for Apple Music. You can only control the music from your computer, as far as selecting the music to be played, and at each HomePod you can individually control volume, and you can also pause the music and go back or ahead track by track from the HomePods. So there is that degree of control but it is not Siri controlled.
One thing about Siri, as others have reported, it is uncanny that you can say 'Hey Siri' at very low volume, even with the music playing very loudly, and Siri will pause the music and wait for a request. That is quite impressive.