One of my friends, who does absolutely no exercise and is what you'd call skinny-fat, bought a Fitbit and was proudly demonstrating how his heart rate (sat with me in the office at that moment) was just over 50. I had to explain to him that considering his lifestyle, either the Fitbit was wrong or he had a medical problem causing brachycardia, because the only person that should have a heart rate of 50 in the middle of the day is an endurance athlete, which he is most certainly not. I measured his HR myself with my watch and a finger - it was over 70.
Fitbit's CEO James Park has said that: "People need to use common sense. It’s not a medical-grade device; it’s a consumer device. In that setting, it works incredibly well." This echoes what they said back in January when the suit was first filed: "it’s also important to note that Fitbit trackers are designed to provide meaningful data to our users to help them reach their health and fitness goals, and are not intended to be scientific or medical devices."
In other words, for Fitbit, it's fine for getting a baseline and seeing how much your heartbeat deviates from that during various activities, but you shouldn't use it to accurately measure your heartrate. Which is a bit bizarre, because they say it's better than, for example, the HRM on the machines at the gym, which measure continuously. A deviation of even 10% is a massive difference when you're trying to get fit. If Fitbits aren't able to measure to any degree of accuracy except by using a previous baseline, they're useless and potentially dangerous.