I did a ton of research on these a few months ago when I was looking for one, and the consensus I got across tech blogs was that none of the consumer-grade fitness trackers are particularly accurate. I looked at FitBit, Garmin, Runkeeper, and Jawbone Up, and every objective review said the same thing. Basically, none of them are accurate but if they help motivate you, great.
I now own an Apple Watch, which, when compared with the readings from a medical grade heart monitor, appears to be very accurate. Heart rate is the primary factor in measuring cardiac output and effort during exercise, so that's why I'm sticking with it.