I currently have a Fitbit charge 2. I've used Fitbit for a few years now.
For me, it offers otherwise hidden insights into my state of health. We're not all totally self-aware spiritualists who can meditate and smoothie our way to good health. Sometimes I rely on my smart watch informing my sleeping has gotten worse, or my resting heart raise has gone up, to indicate I'm going through a stressful time and need to fix it.
Otherwise, it reminds me when I've been sitting still for a long time. Easy to do when you're 'in the zone'.
There's some other stuff I think is specific to Fitbit. Namely, the community side of things? Where you can join a group where you all compete to have the most steps in a given period. The interface to that is great, I think.
And yeah, I lost some weight. But after a while, it just gets annoying. In general, I can't wear watches, bands, or anything like that. After a while, my wrist starts to turn red and break out in hives. I've tried all different brands and its the same result. My girlfriend even gave me her Apple Watch when she switched to another watch and that too, made me break out in a rash after just a week or two of wearing it.
So aside from forgetting to wear my Apple watch, it just causes me rashes.
I'd say it is preference.. some people like them, some don't.
Switched over to the Galaxy Gear S2; I like the software better, I like the ability to choose different watch faces, I like the apps, and I love being able to detach the band at will. I have two bands now, one's my "okay to get sweaty/dirty band" and the other is my "wearing to work" band.
Aside from that I don't really wear it. Notifications on the wrist is nice, but doesn't add that much value. No apps of value either.
I've used Polar HR watches a lot in the past and after usually 15 jogs/bike rides I could estimate my HR pretty darn close (and I would get worse at estimates after a few months and put it back on for a ride or two).