It varies, and there is no way to know ahead of time how it will work for you.

It's not EKG accurate, because it doesn't use electrical impulses to read HR, it uses light reflectivity off veins near surface of skin right under the watch.
That may or may not work great for you.

Most seem to see pretty good at lower HR's where the info is purely for curiosity, not used for calorie burn calc. So sleeping, walking around, daily stuff.

Exercise seems to find several problems. Either not reading high enough as the HR is actually going, missing sections of time as activity causes it not to read well at all, or others where it reads lower level workouts fine.

That's the part that just depends, and you'd have to have an actual HRM to compare with to discern.

I check mine against the fitness equipment at the gym and get similar readings. Again, its not EKG accurate but is a decent estimation, I think.

Compared to my polar chest strap it was over by like 50/60 calories. But my strap is an older polar model so its possible the newer polars are more accurate? Its close enough for me to not deal with the strap anymore, especially since the strap always comes undone when I jog

At my appointment last week they did vitals and I had my Fitbit Charge on. Their machine had my HR at 107 and the FitBit had my HR at 106. I'd say that was pretty close and was quite pleased it was so close.

Pretty good. Ive been testing mine by manually taking my pulse and its spot on.

It can loose it when working out hard but this is better by moving the FB higher up on your wrist. I then use a sweatband to keep it in place.

Fitbit themselves say that this can happen and to stop, let it take a reading and it then uses some calculations to figure out your rate during those blips.

Something to also note. Normally it takes a reading every 5 seconds but when the watch is in excercise mode (by holding down the button for 2 seconds) it changes to checking your pulse every 1 second for more accurate readings.