So far, I like what I see. The new browser feels instantly familiar and I like having all my Live packs in one place. The new M4L modulation devices are very nice to have.
Someone correct me if I'm wrong but I feel like the "new" MIDI editing features have been there for a while. I know that we have been able to select a range of MIDI notes and stretch them relative to each other since at least v6 or v7.
I haven't installed any of the new sound libraries yet.
I spent most of my time grabbing audio from my computer's built-in mic to play with in Ganulator 2. Lots of fun.
The projects that I'm currently working on all opened fine in Live 9, so I'm happy about that.
I got nothing to complain about yet.