I love mine. I've had one since they first came out. The one I have now the Third Generation, which I've had for a couple years...I haven't felt the need to upgrade since (I think they are at the Sixth Generation now).
*WARNING: this is long*
Things I love:
the regular health/activity stuff: activity tracker/heart rate monitor/flights climbed/
waterproof: can even track my swimming laps!
can keep my phone in my purse/pocket as I get all my notifications on my watch...just pull it out when I need to
remote control my phone's camera from a distance, for those wider angle group shots.
can answer my phone on my watch if my phone is in another room (I don't do this often because I feel ridiculous talking to my wrist - a la star trek - but its handy for emergencies). Can also talk to Siri this way, but again, I feel stupid.
Control phone's music (play, pause, select songs, etc)...esp handy when on runs
the timer...i know this is a basic feature, but its probably the feature I used/use the most. It was especially useful when I was timing my bites back in my weight loss phase.
Apple Pay from the watch.
Durability. I don't know how many times I've banged the thing on stuff, and it still doesn't have a scratch on it...still looks like what it did when I got it out of the box.
Things I'm not crazy about:
Face Size. I initially had the bigger faced-one but even when I was obese, it looked so big on my my wrist. I can only imagine how the bigger one would look on me now. With that said, I have the smaller-faced one now, but my old-age-deteriorating eyesight is making it difficult to read. Wish the screen had a zoom function or something.
In-the-box Band Size of the Sport Band: I have smallish wrists and I had to wear it on the second smallest notch (after I lost some weight) which resulted in all this extra band left tucked under. I did just get a smaller after-market one online, but its not as good quality as the original Apple Sport Band.
Battery-life. I think alot of other smart watches can last 3-4 days on a single charge. Mine lasts about 24-30 hours. So I end up just putting it on the charger before I go to bed, so I have never been able to really try out the sleep monitoring function is has. Boo. On the bright side, it doesn't take long to charge it to an acceptable level (20-30 mins) if I happen to forget to charge one night.
The "lift-wrist-to-wake" functionality is not 100% consistent (I'd give it maybe an 80-85%), and the opposite "tap-to-wake" function is sorta annoying (but works 100% of the time). This may have been addressed in later versions, though.
It's not very pretty...especially when the screen off. I mean, it is a smart watch, so what do I expect, but I wish it looked more like a classic analog watch, or something.