On the X90J, I can't say a lot, I'm not a TV buff.Does YouTube not have 120hz vids?
The two big weaknesses are the reflection handling and the viewing angles. But neither of these issues are a problem in my living room.
I don't have any 120 Hz content to test yet.
Image quality is fantastic. I can't really complain about anything. 4K looks flawless, 1080p looks pretty close, 480 varies. Some SD scenes look fantastic.
You can occasionally see some weaknesses in the screen. There are 24 dimming zones. In rare instances you get some blooming. Primarily stuff like white text on a black screen. But it's rare.
Played God of War and it looks really fantastic.