I did use the watch once to see what each HR zone feels like and I thought that was a useful calibration, but as a normal dude where fitness is just one small aspect of my life I wouldn't buy an Apple Watch specifically for that feature.
I'm not saying it can't be helpful with fitness, but responding to OP saying that fitness/health is the primary feature for themselves and many of their family/friends. For me, the primary features are:
1. Telling time
2. Putting my notifications on my wrist
3. Starting timers with Siri
4. Setting up reminders with Siri
Surely there are folks where the fitness features provide the critical marginal feedback that gets them up and moving, to the point where owning vs not owning the watch is a big deal!
But reading the comments here, it sounds like it's very useful for people who are quite scientific about their fitness (HR zones, tracking, etc), and tangentially useful (rings remind folks to get off the couch/stand) for other folks at risk of a sedentary lifestyle. It doesn't all add up to me as "fitness is the main reason many people use the watch!"