I really liked using my Polar H7 chest strap, the app Digifit icardio and synching it to my Fitbit. I felt that really boosted the accuracy of my Fitbit One. I know experts say not to do this, but when I was dieting I ate according to my Fitbit estimated calorie burn. Using this method it worked very well for me. I have yet to see an optical wristworn sensor measure similar to my chest strap or even manual pulse (taken by me or my doctor). For some reason the wristworn monitors tend to read high for me. If anyone knows why, I would love more information on that. I have heard others say they measure low especially during exercise, but for me they seem consistently high for inactivity or activity.
I have tried a Schoshe, and it read high. I love that it works with Digifit though. My Schoshe is kind of old so maybe they have improved their technology.
I have a Fitbit Surge and it reads high. At the doctors office, my doctor counted my pulse as 57 (which is in line with what Polar typically reads) and my surge said 79. I love my Surge otherwise and just use the heart rate information as a scale since it is lower when less active and higher when active. The calorie burn is strangely in line with what my Polar and Digifit devices/apps estimate though I am not really using that number currently. I agree about it not being great fo HIIT and shorter intervals. I don't think the issue is the heart rate monitoring but the way it averages the BPM's. I have a fairly quick recovery rate so hiit workouts look less intense than moderate cardio when I look at the Fitbit report. That is one of the reasons I like the chest strap with the Digifit app.
I just received an Atlas to review. I have not directly compared it to my chest strap, but the BPM numbers seem a little more accurate though possibly still a little high.
I haven't tried the Mio, an online friend in my Fitbit friend feed uses a Mio and recommends it so I have been meaning to try it if I ever get a chance.