Upgrade to miui7 to solve most of the issues. For mic problem boot into recovery and clear cache. It solves the problem.
Coming to performance. It is very fast. Even if it heats due to extended use thre are no lags. Temp never crosses 40 degrees on normal usage. On playing games it may go upto 45 degrees which is normal. Also dont play games while charging.
Battery backup is great too. Easily lasts entire day. Screen on time is almost 6 -7 hrs on constant wifi usage. It will be around 4-5 hrs for data usage. Getting used to miui takes a little time. But over a period of time you will notice it has all the essential features inbuilt which includes:
Security app(antivirus+battery manager+unwanted calls blocker+cleaner etc)
Compass app
Themes app
Backup app
Etc.
Highly recommended phone.