There appear to be lots of negative reviews here but I'm not sure why, maybe people had unrealistically high expectations or they don't fully understand the limitations of the technology. For example a device on your wrist cannot literally know if you're sleeping or just resting but awake, even the significantly more pricey Apple watch can't do that. Likewise it slightly overestimates with your steps but steps are not the only thing you should be focused on, the heart rate monitoring version improves activity accuracy significantly in this regard.
Having had mine for almost a week now I'm very happy with it. The app is very good and the features on the Fitbit itself work well, although I've not needed to use auto sport detection yet. Heart rate monitoring appears to be accurate and most settings can be configured and fine tuned to make tracking even more accurate.
I'm not particularly impressed with the strap, it's just not so comfortable I never notice it. But on the other hand it's not dreadful.
For the price (HR version) this is an absolute bargain, I used to have an Apple watch but found I never used enough of the features. This Fitbit is a perfect balance of features and cost.