After 12 hour of using it.
Do NOT update.
I repeat...DO NOT UPDATE.
It's almost slower than 8.5.5. I mean, I think there are some lack that hasn't been here before.
I got voice search app, which I'll never ever use it.
The clock has weather clock widget, which is nice.
WORST: I cannot open the updater, meaning, next update may need wiping.
Battery was OK so far, nothing more nothing less.
So, if you're at 8.5.5, stay there, wait for miui 9 beta release.
If you are at 8.5.2, go for 8.5.5 is the best option.