I have an Watch SE, GPS and cellular.
The independent GPS feature in the Watch is great!
I use the built-in GPS when I go out biking anywhere I am not familiar.
I use/wear it to make and receive calls instead of always pulling out my iPhone and to use for Siri queries.
I use/wear the Watch for Facetime audio calls or Skype audio calls
The Watch has a neat walkie-talkie feature so I can communicate with my wife anywhere in our home, either over Bluetooth, via my iPhone, or using our home wireless, WiFi network.
Have not tested this using cellular data, yet.
I can play Pandora Radio directly from my Watch via WiFi or cellular without using my iPhone.
I have my Watch configured to give me a decent amount of weather info, any time, at a glance.
Again, without having to pull out my iPhone or iPad or iPod Touch.
I use/wear my Watch when going out for a walk to track my steps, pace and distance walked. It also tracks my route.
I also have it on all day to track my steps doing anything, the amount of time I am up and standing for any length of time and when I am going up and down the flights of stairs in our apartment, for any reason ( trash, recycle, getting mail, laundry day, whenever I venture outside to actually check the weather for myself ).
I really do not know how useful the blood oxygen sensor in the Series 6 is.
Based on info here by other users/contributors here, a dedicated blood oxygen sensor, like the ones you put on the end of your finger, are more accurate and readily available in any major pharmacy and are cheaper to purchase for this task.
The Series 6 ECG feature is only equivalent to one lead of an 8 lead heart monitor and the lead the Watch mimics is the lead for detecting and monitoring A Fib.
There is no way any Watch, currently, can detect any early warning symptoms of an oncoming heart attack or stroke, which is what I really need such a watch/device to do.
Since I have long term heart issues, I can, at least get my heart rate any time from the watch.
And the Watch SE can detect an A Fib event and and send a warning, but that is it.
I use/wear my Watch to monitor my sleep and it acts as my alarm clock, since it is already on my wrist and always close to body or head.
I have my alarm music/sounds and haptic feedback set for loud and really strong vibrations against my wrist.
You can set an overall sleep alarm schedule for the week/weekends and this can be easily changed/modified, on the fly, on a per day basis if you change your sleep schedule any day of the week.
It will default back to the normal sleep schedule the following day.