The Redmi Note 5 Pro ships with Android 7.1.1 underneath MIUI 9. Our non-retail review unit came with 18.104.22.168 Nightly build, which we hope and assume isn't too different from the version that will ship with retail units later this week.
As is the case with MIUI devices, the software on the Redmi Note 5 Pro is more or less identical to what you get on other Xiaomi devices running MIUI 9. The differences are usually hardware specific and can be seen in things like the Camera app (which we will talk about later) but, by and large, the experience is similar. Basically, if you have used one MIUI 9 device, you have used them all.
So, we will skip the summary and talk about the general pros and cons. The pros include a lot of features and functionality built into the OS that you don't get in stock Android. Things like Dual apps (which lets you run two instances of a single app with different accounts), App lock, theme support, Second space (which creates a separate profile on the same phone), and Caller ID (which identifies some unknown numbers and also warns if enough people have reported it as spam).
Xiaomi also does a lot of localization, so you get features like a messaging app that can automatically detect SMS with train ticket info and presents it in a more readable format, ability to filter notification SMS into a separate folder and also an option to copy the OTP easily. There is also a video player built-in that could play everything we threw at it. All of these are useful additions that people have come to appreciate.
Unfortunately, MIUI still looks like something designed primarily for the Chinese market, and just like other Chinese ROMs out there, the iOS influence looms large. This means overuse of white everywhere in the UI, no app drawer and forced square borders around all the icons. Xiaomi also resolutely ignores many of the new UI features in the latest versions of Android; the notifications still cannot be expanded, many of the default apps still require you to press and hold the multitasking key to access the 'Menu' function (something that died in stock Android years ago), the share sheet is straight up ridiculous and you can't press and hold the app icons to access app shortcuts. This is just the tip of the iceberg and listing all the shortcomings in MIUI will require its own separate article.
Bottomline is, Xiaomi has done some good work to make MIUI more relevant and functional to its users. However, it is all built upon a somewhat outdated user interface that needs to be brought in line with Google's UI design guidelines, not Apple's.