I've been using a Fitbit for a couple years now. I have several friends and family members who also use Fitbits and we all compete against each other. I just recently bought my Apple Watch (42mm black sport), and since I planned on wearing both devices, I figured I would compare the two when it comes to fitness tracking. Holy cow is there a significant difference! I had a feeling they would be off by a little bit, but not by this much. Step count on the Apple Watch is as low as 52% lower than the Fitbit! Calories read as low as 90% lower on the Watch as compared to the Fitbit. Without trying a 3rd device, I can't tell which of the two is more accurate. But I'm thinking the Apple Watch is way off. It claims that I only burn 260 calories in an lightly active day. It should read 10 times that, which is exactly what the Fitbit reads. So unless my Apple Watch thinks I'm a slightly animated corpse, I think it's fitness tracking abilities are a bit off the mark.