That the fun part on MIUI, it's allow you to do a lot configuration (not for those hardcore users),
sometime somewhere you will find some bug/error on it and required you to check their forum and seek for solution...
But if you are at stable rom, it will should be fine but if *so happen* there is a bug, you need to wait for a month or two for the official update/patch..
A. The dev rom seems a little bit faster/smoother and more thing to play with....... (example: you got those china apps installed while our Malaysia Stable ROM have it excluded) but also got some minor bug...
B. MIUI is very similar to iOS (although it has Android DNA), and I feel iOS more user-friendly... compare to Android, as you know iOS not allow you to do sharing... everything goes to iTunes...... so with MIUI you have look-a-like iOS + Android DNA (Allow sharing

) which is a good OS to use...
C. It is quite annoy for the weekly update as you need to download 450~550mb of data in order to patch it, and after that done, the file is still in your storage.. if you didn't clear it, it will occupied your tiny 16GB space (to be exact, it only less than 13.36GB usable.)
D. although Mi3 itself have some cons but you really can't complain too much with such price ( wah lau eh, it just RM900 what you want summore.)
I know you sure ask what is the cons» Click to show Spoiler - click again to hide... «
1. Vibration is soft.
2. Speaker not that loud
3. the "Edge" at the phone 4 corner is really .....(fill up yourself), but this can be solved by having casing.
4. when headset is plug in, the speaker still functioning when notification kick in..

(maybe just me but you can try if I am correct or wrong.) Example: when your headset is plugin, you have whatapps open, your speaker still play the sound when you typing/received text..................