Xiaomi has released Android 6.0 Marshmallow MIUI ROM for the Mi 3 and Mi 4 smartphones. V7.2.8.0.MXDCNDB is currently only available for the Chinese region, which means you can install it on any Mi 3 and Mi 4 but being the Chinese ROM you won't get any of the Google apps pre-installed. For that you will have to wait for the global ROM to release. The ROM is stable so should be fit for daily use. However, as mentioned before, unless you can live without the Google apps or know your way around installing them separately, it's better to wait for the global ROM to...
Source: http://www.gsmarena.com/android_60_based_miui_china_stable_rom_available_for_mi_3_mi_4-news-17408.php