I rarely feel compelled to write reviews because the products I usually get are 4-5 stars and I have nothing helpful to add to the review. In this case, I am furious (at FitBit) at how bad this flagship product is for anything involving exercise.tl;dr: heart rate tracking during exercise, lifting or cardio, is so busted it qualifies as false advertising. It was off 20-80 BPM during cardio. Can't even express my shock adequately over that.My background: I'm not an athlete, I'm a regular guy who enjoys lifting, hiking, spinning, and working out in general. I workout 4-5 times a week for 30-90 minutes per session unless I go for long hikes (3-6 hours). I know what my heart rate is when I work out. I've used a FitBit Alta HR for the past 4 years and I've sporadically used a Polar heart rate band around the chest. I know what various heart rate levels feel like in my body and can guess within +/- 10 BPM based on fingers to my neck or wrist. I've had this FitBit Sense for a week after retiring my Alta HR (still working!!!).In the four images, you'll see two different recent workouts. The first image is from 11 November with my Alta HR. I had just finished lifting weights (image 3) and went over to the elliptical and powered it up to resistance 45/100. My METS output was between 10-12 the entire time. You can see the Alta HR took a moment to catch up to my heart rate, but once it did, it was accurate. I really was going hard the entire time and ended the exercise before doing my cool-down. I can sustain my heart rate at 165+ during cardio for 30-60 minutes depending on what I'm doing. I've been able to do that for years. I know what a high heart rate feels like in my chest, in my neck, and on my wrist.The second image is from today, 25 November with my FitBit Sense. I had just finished lifting weights (image 4) and went over to the elliptical and powered it up to resistance 45/100 (sound familiar?). My METS output was between 10-12 the entire time (sound familiar?). Only this time, my Sense appeared to catch up and then it flatlined and reported me for 25 minutes between 82 and 89, which is so laughable it's actually criminal how wrong that is. I tested my heart rate using the machine itself (which is reliable within +/- 5 BPM) and it showed me at 165. I put my fingers to my neck and felt a pulse of around 160-170 (not a doctor or an EMT but I'm a musician and know the difference between 82 and 165 on the metronome).For weight lifting comparison, see images 3 and 4. I chose the same workout for comparison (upper body) and I am WELL aware that wrist-mounted HR monitors struggle with weight lifting because of hand grips changing the circulation of blood to the wrist. You can see in image 3 that I have peaks and valleys - my HR is going up right after doing the lift and it comes down during the 60-90 sec recovery. The Sense (image 4) struggled mightily, showing what appears to be a more "consistent" reading across but missing the peaks and valleys that weight lifting leads to. For more context, I was benching 185-225 during the first 8 minutes where it saw me in fat burn and I can tell you, my HR was well above 120ish after the set at 225.All of the other FitBit Sense features, so far, appear to be great. I like getting notifications on my wrist that I can choose to see, like having my various messaging apps notify me. I like the Star Trek Console clock face - kudos to the developer because it makes my nerdy side smile. I like that it measures SpO2 while sleeping. I like that I get excellent sleep tracking that compares very well to my old FitBit Alta HR. I like that I can input other metrics like my weight or stress into it with the Sense. I haven't yet tried the ECG or EDA, but I expect those probably work just fine too. The visual of the watch on my wrist is nice and people have already complimented it.But the fact that in November of 2021, more than a year after launching this flagship, $300 product that is meant to compete with the Apple Watch, Garmin Vivo series, and a suite of decent Samsung watches, more than a year after MANY online complaints about the heart rate monitor not working during exercise, for this to still be a problem indicates that something is actually wrong at FitBit itself. I work in a medium-sized corporation and if our flagship product was actively FAILING to do the ONE most basic function the entire company was LAUNCHED on, people would be getting fired left and right. I guarantee you that the engineering department likely has not received enough negative feedback about this. I'm sure this review will never be seen but it sure is making me feel better writing it.It's not only laughable, it's actively pathetic that a FitBit heart rate sensor in 2021 cannot accurately record a heart rate during exercise. My Alta HR, from 2017, was accurate. This thing was 20 to 80 BPM off. That is STUNNING. That is EMBARRASSING. That is FALSE ADVERTISING. No athlete, no gym rat, not even a regular person wearing one of these should even remotely trust any of the other readings the sensors provide if the most basic, most simple, most well-tested sensor since the mid 2000s, a heart rate sensor, isn't working right on a $300 flagship product. I mean it's so inexcusable I can't even believe I'm writing this.And real quick - the FitBit customer service representative claimed that "I wasn't wearing it right" (per FitBit's own instructions, I've been wearing the Alta HR above my wrist by two finger widths for 4 years and did the same with the Sense this past week). Then, he told me that I couldn't "claim" the warranty, after one week mind you, because I DARED to mention that I did a comparison with a Polar HR band. Somehow, mentioning that you know what you're talking about and can see the faults means you don't get a product replacement? Literally that's criminal behavior and FitBit should be shocked and embarrassed by that. I don't know who they contracted their customer service out to (and I won't indicate from what region this person likely was because there's no need for that), but wow. I've never had a worse customer experience over the phone in my LIFE. And I've had to deal with cable companies!But lemme give a quick shout out to my man Joshua, the Amazon representative. Five stars. He heard the whole conversation and he couldn't even believe the FitBit customer representative was actively avoiding trying to provide a replacement. And so he had my back and processed a refund.