Finally got my hands on a Series X, it's kind of crazy how exactly the same, but faster, it is compared to the One X. MS is definitely going for a unified feel across consoles.
Transferring games was foiled a couple times due to them needing to be redownloaded completely for Series X features, so now I'm over my data cap
But I did pickup a couple years of XBL Gold, then converted it to Game Pass, so that's cool. Checked out Flight Simulator and it's as gorgeous as advertised. Other than that, I've only tested previous gen games, which all work smoother/faster.
Oh, and being able to quick resume multiple games is fucking amazing.