Bought mine to play computer games above all.
Games in 2560x1440 are scaled up excellent, 120Hz works and I don't know any input team even in very demanding games like Vermintide 2 compared to my 165Hz Acer Predator game screen. G-sync does not work with my 1080ti, but according to Linus Techtips it will work with the 2000 series and 1660 series. Reasonably well with the upcoming 3000 series. HDR HGiG is available on the C9an, a quick look at the Metro Exodus gave nothing further with the setting but I haven't tested it thoroughly.
HDR on Horizon Zero Dawn provides either clogged shadows or heavy concealment in dark areas. The picture will be best without HDR in YUV, limited tone range on the PS4an and low black level on the TV. RGB and full tone range on the PS4an and high black level on TV also give a good picture. It's a matter of taste.
Borderlands 3 looks clumsy and three-dimensional played on PS4 with HDR. The black level on the TV needs to be low and the HDR luminance in the game is increased to max.
As a black and white photographer with Ansel Adams as a role model, I am particularly interested in high contrast range with coverage in light and dark areas. HDR works well and ranges from the whitest white to the blackest black. Metro Exodus requires lowering the old to about 0.8 in NVIDIA's control panel so that dark areas should not be gray. Worth mentioning, though, is that the OLED panel's lowest possible brightness is much higher than I expected. This means that it is not possible to obtain all coverage in dark areas with subtle gradations. The luminance difference between a turned off pixel and one that is turned on at the lowest possible brightness is thus high. It can be tested with this video: https://www.youtube.com/watch?v=IM8aSFAOAkk
On the video, the OLED panel does not respond until a value of 15, my LCD computer screen responds at 18. If you have a TV that responds to a value equal to or less than 15 and is looking for coverage in dark areas, the LG C9 do not give you anything other than carbon black areas where the pixels are off. The panel is good, but the road to Ansel Adams is still long.
A side by side comparison with Samsung's Q90R would have been very interesting. Unfortunately, the comparison videos I have seen online between the C9an and Q90R all make the same mistake and do not set the TVs to the same brightness, which makes the comparison meaningless. It is clear that the Q90R will have dark areas covered if it is set to its 2000 nits and the LGn to its 750 if the person who produced the comparison video sets their camera after the C9an. In addition, the cameras they use do not have enough contrast scope to compare the TVs. The only way to know for sure is to make the comparison yourself.
An important difference between these is that the C9an has HDMI 2.1 which can deliver 4K in 120Hz with minimal input lag. The Q90R has HDMI 2.0 but can already run 4K in 120Hz with low input layer but of course with compression fees.