I bought the Mi3 after a lot of deliberation 2 weeks ago. I currently have a Windows phone and always keep a spare since I go back and forth from Bangalore and Seattle.
My previous Android phone was the Google Nexus. I have had a iPhone 4S as well.
My overall basic impression: This is not the phone for me. I am ready to sell it to anyone that wants it.
I had a chance to see the phone in action 3 weeks ago when 2 other folks at the accelerator bought it. It is EXTREMELY light. It has a gorgeous display and I had heard so many good things about it that I was tempted to buy it.
It is a very well made device. Fast and sharp, if you in the market for an Android phone and have bought into the Google ecosystem (use Gmail, Google Maps, etc.)
The 3 most important things to me are consistent access to email (I have an Exchange and a POP3 account), long battery life (my other phones dont last an entire day) and reliable phone (good signal, loud enough with a headset). I use very few apps except to post to FB and Twitter and some minimal reading (Feedly).
Unfortunately these are the only things that this phone absolutely does poorly. In fact it is so bad that I am tempted to go back to my Galaxy Nexus (which is very slow).
First: email. As I mentioned, I have Exchange and our corporate policy requires encryption of the phone to access email. That does not work with MIUI. After 5 restarts and 4 hard resets, I still dont have my Exchange email. Which also means my calendar is not available. It is a known bug according to Xiaomi and there is no ETA on the fix.
The work around is I downloaded another email client, which seems to work, but my contacts and calendar on Exchange still dont sync. That absolutely is a deal breaker for me.
Second: Long battery life. It is much better than my Nokia 820, but the phone heats up quite a bit when you use it for over 2 hours (especially when you use maps). It is definitely much hotter than my 820 or the Galaxy Nexus. The battery has not lasted an entire day of normal usage. Disappointing.
Finally: I need a good phone. I tend to be on calls for over 2 hours daily. This is very weird. When I call my voicemail, the screen freezes. The phone still works, but the screen just wont turn on. It is absolutely impossible to do anything after that other than restart the phone. I had 7 voice mails to go through and they are still stuck without the ability to delete them.
If you need a good phone and dont work for a large company with Microsoft Exchange, etc. this would work, but there are cheaper phone that do the job as well.