And I'm pleased to say, it isn't bad.
Switchgear is relatively nice, save for the cheap plastic paddle shifters that they're using instead of the real magnesium ones they used in ATS/CTS. Leather on the doors and dash are of very high quality, and the hard plastic on the lower dash and doors are textured nicely and don't feel like rubbermaid quality plastic like CT6 and XT5 use. The overall quality of the dash is better than its XT5 sibling as well, in that the XT5's interior feels like if you push hard enough, you can break the entire dash and rip it in two. XT4 is better in that regard.
There are some cons, with the paddle shifters being one of them, but my biggest issue is with the fit and finish of the dash itself. It's like Cadillac just can't get it right, despite being called out for it on nearly every car.
Look at this:
"Hand-stitched", huh? Were they drunk?
Outside of those two issues, XT4's interior as a whole is a much, much, much better effort than the CT6 and XT5, that's for sure. There's room for improvement, sure, but as it sits now the interior is not what's holding it back right now. I'd actually say it's a selling point.