My Personal Review: 

I am using Samsung Galaxy note 3 Neo and Redmi Note. First, I have tried it on my Note 3 Neo, it works but it is a little bit lagging when performing tasks on MiKey. Theoretically, it works out on all the android phones as long as you have installed the MiKey's application. But there is possibility that it might not works on some phones. 

Second, I tried it on my Redmi Note, it works perfectly nice! 

From my own opinion, it is good to use for emergency purpose. For example, it u need to call your closed one when you're in a trouble, you can call him/her by just pressing on the MiKey without turning on your handphone to search for the contact number. It is incredible awesome! 

Contact me at Shirley's Page if you're interested to this product! :)

Description of MiKey:-

What is a Mi Key?
Mi Key is a clickable button that can be plugged into your android phone headset jack for you to take quick pictures, turn on the flashlight, record phone calls and so on, all without unlocking your phone. You can program up to 10 actions for 10 different click types using dedicated app.

Video : https://www.youtube.com/watch?v=PAz5-WBJcBA

Is it for MIUI/XiaoMi phone only?
A2: No, theoretically it is compatible with all android phones as long as you have installed the MiKey Apps. But we do not rule out the possibility that some phone might be incompatible.

Where can I download Apps for Mi Key?
Official Chinese Version - http://app.mi.com/detail/59090 , 

3rd Party Translated Version - http://xiaomi.eu/community/threads/mikey-app-1-6-2.24220/