The Xiaomi OS beta testing has been in the works for some time now, and now, two weeks before it ends, the software is available fr download for everybody. You can test it on you Xiaomi device for yourself.
The OS works for the:
Mi 2, Mi 2S, Mi 3, Mi 4, Mi 5, Mi Note and Mi Max.
On June 17 they should add support for the Mi 4C, Mi 4S and Redmi and Mi Note phones.
Speaking about changes, the UI in MIUI 8 is a lot diffrent from the UI 7. This include new notifications, new animations and many more. You can also save some battery due to new power-saving improvements.