Mi Band 2 is a fitness band / smartwatch from Xiaomi. It is a good band & acurate too for sleeping, walking etc. but if you want the exact heart rate then don’t go for it as the heart rate sensor dpes not work every single time & you’ve to wear band properly & tight if you want to measure your heart rate accurately else it’ll not work. So that’s all up to you whether to buy it or not. As an alternative you can also buy Mi Band HRX edition which completely omits the heart rate sensor & also give you better battery life.