Overall, a great experience.
It has the typical section-based On-Screen-Display, which is controlled by 4 touch-sensitive buttons. It takes four taps to be adjusting the brightness (I settled on Contrast 50, Brightness 60 and never change it now. Also disabled all the post-processing features and set the colour temperature to "Normal"). It has a nice so-called "Dot by Dot" mode (aka 1:1), meaning that you can show a less than 5K signal with black borders rather than being upscaled.
Based on the advertised specs (oddly they say 6-bit panel, but no such 5K panel seems to exist), there's some question about the image quality. But as someone who's actually used it, I have no complaints about any aspect of the image quality. This is from someone who, although not a pro photo/video editor, is fussy about just about everything when it comes to computers (I was already seeking out IPS monitors in the mid 2000s).
Haven't used the LG UltraFine personally, but compared to the 5K iMac, this monitor doesn't feel substantially different.