It’s a nice update in some subtle ways. Things seem a little snappier in SOME ways that I can’t quite describe (though still behind Pro Tools and Reaper on my system), and it’s completely stable for me. Nothing except NI Replika was blacklisted, and I have tons of 3rd party plugins, so I didn’t experience any issues there. It feels largely the same as 8.5 with a slight increase in CPU efficiency (still has a significant way to go to be on par with Pro Tools or Reaper, though - my studio mate has the same experience with Cubase on his Windows 10 machine). Feels a little more “well-oiled” somehow, which is cool. Mixer undo is fantastic! Very, very happy about that. Well done on those fronts! Haven’t had any use for the Sampler Track or FreqEQ yet. The lower split window is not needed for my workflow/setup, though it can be cool to use some of the editors there at times. Also, the lower transport is not needed for my setup.
The Mac GUI lag is without a doubt still very present for me (worse in some ways), and is as maddening as it ever was. I’m going to talk to Steinberg about it and try to figure out why some Macs don’t do it and some do (like mine)…it’s close to being a deal-killer at this point, when you’re someone like me who works at a breakneck speed (and was used to the crispness and quickness of Pro Tools on the same OS/computer before I came to Cubase). Re-installing Yosemite and everything from scratch again is not a realistic option time-wise right now, have too many deadlines.
Sometimes the “period” button on the number pad doesn’t like to work for some reason, only in C9 – I use that to input bar or TC position, and I use it a lot…so I’ll need to find another key to do that with temporarily.
On my Mac it still simply isn’t as crisp and fast as Pro Tools or Reaper overall, in a number of ways (not just the GUI). Things like the saving time, the time for the app to close after big projects, and more. Apple is not to blame at this point since the other DAWs that I use on my Mac are super fast and snappy. 
So, nothing was broken for me, and there are some subtle, nice things about the update. I hope they work on the issues I’ve mentioned here for upcoming updates instead of plowing ahead with any new plugins, etc, as well as getting automation editing up-to-date, and a number of workflow improvements that have been mentioned a lot here such as the still-very-clumsy clicking in the ruler, inconsistencies between windows, Warping directly on the project page, giving us the arrows back at either end of window locators (why on earth was THAT taken away? So weird!), etc. These are not things that are trying to be like other DAWs, they are things that are great, efficient, and useful for universal reasons, which is why the majority of other DAWs incorporate them.