As a fitbit Charge HR user, I did my research before hand. I wanted something to help me monitor my workouts, and keep an eye on my pulse rate, to keep in a a range. As well as to motivate me a bit more. Which it has.
That being said. I did a lot of research on which one to get, the biggest complaint of the (at the time brand new) device was the accuracy of the heart rate monitor. There were even medically trained people saying that the method and how it is held in place is inherently inaccurate. They recommended a nearly $1000 device for proper monitoring of a heartrate, (a chest wrap one) if you needed that level of accuracy, but for casual exercise, the fitbit did the job, and unless you wanted to have medically accurate numbers, they were "good enough". Even Fitbit itself said that it is not for medical purposes, and that a number of things could cause inaccurate readings, nor did they make any promises about the accuracy, even going so far as to say to seek medical assistance if you suspect you have heart issues. a lot of legalese for sure, but they never promised 100% accurate results. They don't even market to hard core atheletes, but more to the enthusiast level. Which has different levels of accuracy required.