Before I begin, I should make it clear that I don't do many of the more 'advanced' features that many on this sub do. I don't watch 4k content, I don't (generally) do 5.1 and I don't do HDR or anything like that. I also flashed the Nougat beta onto my Mi Box as soon as I got it, so my experience is based on that transition
There are a lot of people here and on the Xiaomi forums that have been issues with Oreo, so I thought I might share my experience, now that its been out for a while.
TL;DR Its good, better performance and better interface (IMO).
Firstly, I was skeptical about the Oreo's interface, but I've grown to enjoy it. Luckily the two main apps I use (Plex and Netflix) both have excellent support for channels, meaning the home screen is far more productive than it was when it was just a simple app drawer.
I also noticed a big performance jump, (likely due to the improved processor firmware). Rather than it stuttering constantly when playing through video, it was now much smoother.
About the only 'issue' I have is the 'feature' which means you can't use USB debugging and a ethernet adapter at the same time, which is a pain for what I like to do.
Obviously this will not be representative of all Mi Box owners, but if someone only used HD video then it might be helpful.
I will admit I was wrong about Oreo. I thought it looked terrible initially. However after using it on the Shield, I think it's an improvement. Being able to use the app Sideload Launcher is great for creating channels with apps.
The apps grid that comes up when you click apps is quicker to scroll through as well.