I have a Garmin that is the equivalent of a FitBit. I've used the Garmin running GPS systems for years, so it only seemed natural to get their activity tracker as well.
As a registered dietitian and certified group fitness instructor I can tell you that the calorie counts on these things are quite inaccurate. The heart rate readings are good though, and if you properly calibrate them, the distance readings are good as well. But you do need to calibrate it for your own stride length. The calories though, those aren't accurate. They give a general idea, but often estimate on the high side.