For fitness tracking, I think there are better options, but if you want a wider set of features the Apple watch is good.
I've used a WHOOP for the past 9 months and think it's tough to beat as a pure fitness and sleep tracker (I've used a bunch of stuff over the past years including a Fitbit, a couple Garmins, and the Apple Watch 2, but never stuck with any of them longer than 6 months). The app gives you a ton of data (RHR, HRV, decent estimate of calories burned, sleep tracking), the recovery levels help you calibrate how hard to train on a given day, and it can also help you figure out how different habits affect your recovery (e.g. I've learned that what time I go to bed is hugely important to my sleep quality). From a functional perspective, I like that it's very comfortable and low profile, durable as hell (I wear it during crossfit workouts and never worry about damaging it), and that you don't have to take it off to recharge the battery (this is my biggest knock on the Apple watch - it takes about 3 hours to recharge every day so you sacrifice a lot of wear time or can't use it as a sleep tracker at night).
My wife has the Apple Watch 3 though and loves it. She likes the more simplified activity tracking (e.g. daily rings, hourly movement goals) and also uses it for a lot of texting and calendar stuff during the day. She is due for a replacement and we have been looking at the 6 vs. the SE - the real difference is the Blood Oxygen and ECG apps which she is unlikely to use so will likely go with the SE. The 6 also has the "always on" display as opposed to the other models where the screen turns off which I guess is just a matter of personal preference.
