Companion app auto-calculates the HR zones 'incorrectly' (0-65, 65-75, 75-85, 85-95, 95-100), but you can always enter your own values. Otherwise, it's great.

The Bolt itself is fantastic. I love mine. I don't know if it will pair with the FitBit, however, though it'll pair with just about anything with BT capabilities, so I wouldn't expect problems there.