The long-and-short of it for me is: mehhh. The first thing I did was turn on the Use Legacy “New Dialog” in the General tab of the preferences so I can avoid the highly annoying New Document dialog box that cuts off the Margins & Bleed entry areas to make room for giant useless icons for standard documents that used to live in a tidy little drop-down menu.
The new “Spectrum UI” is a huge leap backwards. You used to be able to adjust the brightness of the entire interface with a slider in the prefs; tweaking it just to your liking. Now you have four options: Dark (too dark for me, and too much contrast), Medium Dark (can’t decide if it wants to be dark or light and fails at both), Medium Light (which has no contrast at all and makes the entire interface look like a giant gray box), and Light (which is bright but useable).
I like the “flatter” interface, but it’s nothing to write home about.
Since David Blatner did a whole lot of work writing it up, I’ll point you to his review at InDesignSecrets.
Since you’re heading over to InDesignSecrets, take a look at these tips while you’re there:
Adding Alt Text to Images With Object Export Options
Naming Items in the Layers Panel
It’s nice to see Adobe updating InDesign regularly, but I’m starting to feel a bit neglected with the lack of new features, bug fixes and overall speed increases.