DH and I both have them (earlier models). Love them! We both do quite a bit of exercise, and love the tracking feature (I got mine through my work health insurance, and the more exercise I do, the less I have to pay for - my watch will be free, I get Amazon Prime for free etc etc, but that's the insurance rather than the watch). It's really motivated me to do more.
The calls, texts, calendar reminders, What's App, Messenger alerts are all brilliant, I can quickly check if there's anything I need to deal with, and often reply via the watch rather than phone.
Apple Pay via the watch is handy.
I charge mine overnight, but I do the same for my phone - no biggie.
I don't have the 3G version, I didn't feel the need, quite happy without it. I like the newer version, slimmer face and bigger screen, but I don't think it's worth that much more than the series 3.