Well, my thought is somewhat a mixture.
Pros:
1. Clear design and fewer icons. I hate a row of icons on bottom. This new one looks cleaner than previous one. (I have turned Apple Music off and Connect off in Restrictions)
2. Up next lets music playback more like iTunes. I was hoping this feature could go live on stock music app long time. Now if I want to listen to a song while not switching to a totally different queue, I can just "add to up next".
3. Mini player is also my favourite. Although, yes, iPhone one is not as good as iPad one.
4. Recently added is great for me, especially I want to quickly listen to songs just imported to iPhone/iPad.
5. Ability to jump to album/artist without leaving current view is also great.
Cons:
1. Constant banner if "showing only offline music" is on. Annoying.
2. UI animation speed is too slow. I hope it can be faster, cause before animation ends, I can do nothing.
3. Somewhat imply me to go to Apple Music, while what I can find in Apple Music is not what I expect, or I cannot find a song I want to listen.
4. Smart Playlist cannot display four-album-artworks combined picture as artwork, while desktop iTunes can.
5. Search feature doesnt support fuzzy search. (Which should be a problem even before iOS 8.4)
For alternative, I use Marvis. Really powerful and highly customisable. For those who don't want customisation, default set is also really great.
Ecout is not easy to use. "Listen" is somewhat not intuitive for me. "Muzik" has too many bugs. "VOX" is really unstable, plus quitting app will stop music playback.