I love not having to carry my big cellphone around all the time. It has many apps at the convenience of your wrist. I haven't even finished exploring all the gadgets in the watch. I mainly bought it because I wanted to be able to go on runs and workouts and be able to listen to music or answer call without having to carry my cellphone. Everything else is just an extra perk for me. The only negative for me is that I prefer the look of round watches rather than square ones. I was originally going to get the Samsung 3 series but I found that a lot of what makes that watch great would not be compatible with my iPhone. If Apple started making a round watch, that would make it a perfect 10.
The sound quality in cell-phone calls is amazing!
Bigger screen (pre was series 0, btw)
Always on - is pretty useful
The accuracy of the oxygen level sensor is doubtful, but why not?
ECG - will see.