Never had an Xbox one, but enhanced og xbox games on Series X have been pretty cool but with some occasional drawbacks.
The actual in-engine graphics look perfect a lot of the times. But things fall apart when watching CGI movies in a lot of titles. The upscaling makes cinematics look very low res. A lot of the in engine graphics can look super angular when blown up to 4k too.
Sone games look perfect and seem made to be played at high res though (like anything by team ninja or Panzer Dragoon Orta). Others look pretty terrible and the higher resolution exposed problems in the artwork (like psychonaughts, Morrowind, or Otogi).
Overall game coverage is spotty too with some notable omissions (lacking JSRF on BC should be considered a crime at this point).
Overall, I think the games do look pretty great but I wouldn’t say that they justify a new hardware purchase unless you grab a console for cheap. If you want to play modern games plus Xbox 360 enhanced titles on it too, then it starts to make more sense.
The BC games run well though and don’t have hitching. Things have improved massively since the og Xbox to Xbox 360 BC days.