My girlfriend has a Fitbit Charge HR, it's been almost two months now. It's not accurate for intense exercise. If you're tooling around the local bike trail it will be very close, if you're doing intervals or hill repeats it will be somewhat erratic. Of course, a lot of the accuracy depends on where you wear it, how much hair and pigment you have on your skin, etc.
The automatic resting heart rate feature is pretty neat, though, and if a reliable HRM for exercise isn't your goal it's a great device.