Honestly not a big fan of control center. Don't like 'cards' for music and such; I thought it was fine as it was. Took me awhile to figure out that 'raise to wake' was auto-enabled, and I couldn't figure out why my phone kept waking up even though nothing was being pressed. Notification Center is ok.
I'm trying to figure out why Wallet is an option under iCloud on my iPhone, but not my iPad Air. At work, I updated some info in the wallet app on my iPhone, assuming the iCloud option would sync said info to my iPad; it did not.
Music app is an improvement for me in some areas, not so much in others (no song ratings?)
iCloud used to back up my purchased books to iCloud backup in iOS 7 and 8, something that wastes my purchased data (and at the time, my free 5GB) because the books could already be downloaded from the purchased section. Backing the whole book as part of an iCloud backup was redundant and a waste of space. It would be like backing up an entire purchased app instead of only its data. That problem seemed to be fixed in iOS 9, as the size of my iBooks backup was rather small, only backing up my non-purchased books and PDF's (as it should). Now, it seems the issue is back as my iBooks backup has increased to several times it original size.
Another problem with iBooks, it seems updates to books don't sync back to the desktop app. I always get a "iTunes has found purchased items" message when I update a book on my device, which leads me to believe its syncing the update to iBooks. But when I open iBooks on the Mac, the updates have not transferred. Also, I'd like to be able to see what updates a book contains on my iDevice, much like you can see what updates an app contains.
The new photos app seems really cool (especially since all of its detection is being done on the device), but it's taking forever, and it appears each device has to go through some scanning process before the features sync up. And I'm assuming we will have to wait for Sierra on the Mac before any features sync over to the desktop.
I do like the News app, I'm much more likely to use it now, and I think the notifications it has are cool. The new home app actually makes me want to use some devices with it now. I like Apple Music's interface and integration? 3D Touch is better utilized. Siri integration with third party apps should be great as time goes on...
Quite a mixed bag for me.