I wrote this in a reply to a previous mi band thread..

So I've got this. I've had a mi band pretty much since release and I really like it. Here's what I've found with the heart monitor;

While Running;
-If you start a running activity, the heart rate monitoring will be continuous until you stop the activity. It will give you an average heart rate at the end of the run. I run my preferred run tracker (runkeeper) at the same time. At the end of the run I edit the runkeeper activity to add my average heart rate. There is no provision to extract all the data, or see a graph, you only get the average heart rate. You need to have your phone with you while you run.
-You can set an alert to warn you if you reach a particular heart rate. I set it too low when I tried it and it was buzzing every minute or so and was very annoying. I read Chris Ryan's SAS fitness a couple of years ago and it suggested that you could exercise vigorously until hitting a target heart rate, and then 'rest' for a bit, before bringing yourself back up to your target heart rate. I need to do some research as there's a lot of conflicting info about heart rate zones, but this could be a useful feature.

While Sleeping
-There is an option to sporadically check your heart rate while sleeping. This promises to provide more detailed sleep tracking, at the expense of a bit of battery. There is no visual difference to the sleep graph.

User Activated
-You can use the app to ask the band to check your pulse whenever you like. I've had a few erroneous results but on the whole it's accurate.

If you are not going to run with your phone (and you are not bothered about more accurate sleep tracking) I would suggest either not upgrading or buying a generation 1 mi band. You can check your pulse using your smart phone camera and an app (I use instant heart rate for android, but there's a plethora of options) Not upgrading will afford you better battery life.

I have this, it works very well. Information on sleep tracking, daily steps taken, distance walked, calories and grams of fat burned are pretty good.
However points to consider:
The HR is on demand only
The calories and fat burned via any exercise that does not involve walking/running such as resistance, HIIT exercise cannot be recorded due to type of HR sensor.
For the price it's extremely good.

I have one of these - great for the price.

Ordered one this morning, getting cold votes - am I missing something here????

These are great even if you just wanted a silent alarm to alert you without disturbing others

I also have one but find the heart rate monitor extremely inaccurate . Good for a pedometer but that's about it. Great battery life though, lasts months

I had this. Very inaccurate. Not constant monitoring of heart. Uses app for you to press for heart rate. You can presses twice in a row and get wildly inaccurate readings. Got a refund through PayPal as gearbest would not communicate.
After sync the time was 20 mins ahead of the app. Complained but apparently not an issue.
I wouldn't buy again.

HR is constant in running mode only (Android, iPhone app to be updated). It won't give you a graph but an average at the end of the run / walk but during the exercise its have a live real time view.

I have two of the old version of these.

The pedometer isn't very accurate and one of them died completely after a couple of months. It kept a charge less and less each time until it did. The other is still going.