I've used the weekly updates of MIUI V6 on my Redmi 1S WCDMA for several weeks now.
I came from MIUI V5 v50 and updated fom SD-card using TWRP 2.8.6.0. If did a factory reset to get a fresh start.
My biggest problems until now are:
Freezes of up to 20 seconds (Facebook, aCalendar, Candy Crush...