I purchased this when my Charge 2 started having issues tracking my heart rate during cardio/peak exercise and no longer showed my stats "live" in the phone app (I had to track by looking at the watch each time or forcing a sync). I had a few issues getting the new one to work well with my heart rate too but once I figured it out it worked well. I'll go into what worked and what didn't at the end of my review, because without the heart-rate feature I would rate this a 2... but WITH it, I love the thing and give it a 5.Spotify: I have not used this feature and have no interest in it. I tend to watch streaming shows during my workouts because it's 8,000 degrees outside most days in Texas and I would rather be inside with the AC.Swimming: I am very excited about this feature, but with the current COVID situation I haven't gotten to test it out yet. I like the idea of being able to track my swims as well as my runs though and can hardly wait to try it out. That said, I HAVE worn it in the shower a few times because I forgot to take it off and it survived that with no issues.Comfort and Weight: This thing is significantly lighter than the Charge 2, and the band is easier to change out. It makes it more comfortable to wear while sleeping.Sleep Tracking: This is one of my favorite features, and I like the addition of the oxygen variation tracking. I was hoping that the oxygen tracking would be more than just sleep, but hey, it's nice to know I don't have sleep apnea. The feature basically tells you if you have "high" or "low" variations in your oxygen, and for the most part mine are low. The rest of the sleep feature tracks Awake time, REM time and Light vs Deep sleep. It provides an average trend for your sleep over time, plus a "benchmark" and a "sleep score" so you know how you did. It's helped me be more mindful of my sleep and improve the quality.Heart rate: This tracks my heart rate well and tends to align closely with my treadmill's tracker. I had trouble getting it to read me consistently at first at higher zones, but keeping the watch just below the wrist bone one notch tighter than I normally wear it + using the phone app to look at my heart rate rather than regularly checking the watch helped. That, and I use the watch's exercise app to track the workout now rather than letting it auto-detect like I used to. No issues since.Cardio fitness score: I like knowing where I rank with average and seeing my numbers slowly improve as I get more fit. I wish that there was a feature I could turn on to track pregnancy, however, because your resting rate goes up significantly when pregnant and it lowers your score.Female health tracking: I used to use Fitbit for this. It works well if you keep up with it, but I have another app (Ovia) that I preferred to use once I was trying to conceive.Active Zone Minutes: This feature annoyed me at first. You get twice the minutes for being in Cardio or Peak, and I didn't care to know how many "extra" minutes I got, I wanted to watch my "actual" minutes of workout. Well, at the end the app tells you how many "actual" minutes you get for the workout anyway and as long as you somewhat pay attention to the time (or use your treadmill to track it) you'll know while you're working out too. I get why this is a feature - being in cardio or peak is better than being in fat-burn for calories and workout quality so it encourages you to beef up the exercise, but I was being cranky and resistant to change.GPS: If you do go out and run, GPS eats your battery for lunch. But it's cool to see where you've beenBattery: When not using GPS, it's awesome. SO much better than my previous fitbit.Texting and Calendar: I like having the texts in a block instead of scrolling by like on the Charge 2, and the Calendar is always nice. Sometimes you're not with your phone and a tap from your watch to remind you you've got a meeting or an appointment is really handy.Hourly reminders to move: I used this less when I was in the office every day but now that I am pretty much perma-telework it's great. Even if I'm in a meeting I can get up and move and I feel accomplished when I get 9/9!Customizable clock face: Being resistant to change, I liked that I could put the "classic" watch face on my new Charge 4. I appreciate being able to see my step count, heart rate, date and time all at the same time and none of the "new" ones did that for me. The closest one had a sort of circle thing that tracks how close you are to your step count goal but that's not a feature I care as much about so I went back to my old ways.Automatic exercise recognition: I used to use this all the time, but when the Charge 4 was also losing my heart rate, I started using the exercise tracking apps and that seems to help. While it does auto-recognize my exercise that only works if it can read my heart rate properly.Fitbit losing my heart rate:So, when I do my treadmill runs I like to be able to have the app open and watch my count and heart rate there rather than having to look at the watch over and over. When my Charge 2 stopped doing that, I got into the habit of checking my wrist regularly only to find that my heart rate had stopped tracking and I wound up spending most of the workout adjusting the watch to get it to read. It was disappointing to do the full workout and not get "credit" for that work. Yeah, I know, I exercised anyway but I like seeing the trend and the data.So, I bought the new Charge 4 to fix that issue, only to find that it had issues tracking my heart rate as well. After much finagling, I figured out that my new habit of constantly checking my watch was the issue. To resolve that problem, I eventually found that if I used the "exercise" watch app and set it to "treadmill" plus use the Fitbit phone app to watch my heart rate rather than the watch, I have 0 issues getting it to continue tracking my heart rate. I also tighten the band one notch each time and leave it snug just below my wrist bones so it doesn't move so much, even when I sweat.Here's the stuff I tried that didn't work, just to save you some time:1) Turning the watch upside down2) Moving the watch higher on my arm and tightening it. My watch tended to slip around more the further it was from my wrist3) Moving the watch lower on my arm and tightening it. My wrist bends made it lose my heart rate less frequently here, but it still lost it