I've got a Fitbit Alta, but it's not waterproof (which is my major annoyance with it). Other than that, I really like it. It counts steps, calories (but I think these are fairly arbitrary TBH), km, I'm not sure if it has to be worn round wrist though, rather than clipped somewhere (I think the Fitbit one can be clipped).
It automatically tracks when I've been for a walk, run or cycle, and then it has a load of extra activities that I can log manually (so I log my yoga, if I've been swimming, playing tennis, gone to circuits class, whatever).
I like it to track my activity levels, it's a bad day when I've not managed 30 mins of activity. Its also does my sleep, but that's really inaccurate. It can also track what you're eating and drinking, but I'm not trying to lose weight, just keep active really.
I also really like the challenge aspect with friends, which makes it slightly more sociable. Haven't regretted it at all (had it just over a year).
My DH has an Apple Watch, and in terms of fitness, it's pretty similar, but of course has all the other functionality too (and latest models are waterproof I think). I'm not prepared to upgrade at the moment though.