I'm still on S2 and see no reason to upgrade. Initially I wanted for the ECG and O2 measurement but then when I researched it more those 2 are totally useless for me. I'm not in the risk category (60+) and overall its more marketing thing. Anyone interested with the topic, you can google doctors talking or measuring ECG and talking specifically how not useful it is at this point. So, after a while I realised there is no point for me to upgrade until there will be a feature that is truly useful.
As of now, all these fancy ones are just mainly marketing gimmicks. Maybe one day the watch will be your personal doctor but for now, its not there yet.
Like ECG for example - Watch has only 1 measure point whereas any hospital has around 6-7 (or more). Giving user false positive also creates issues not only for the person but also for the health care system so overall its not a reason to upgrade. Those interested please research it as you will be amazed how much complex the issue truly is and not what Apple wants us to believe.