If I find the S0 useless, would that judgement be any different had I used an S3 instead?
Things that improved in later generations over the S0.
- better battery life
- better sensors / more types of sensors
- better app performance
My main use for the Apple Watch is as an activity tracker and motivator. For which the S0 worked very nicely. The later generations are smoother to use, but if the activity tracking functions of the S0 don't win you over, it's not like the later generations have completely new functions the S0 doesn't. If the sensors on the newer models, like compass, altimeter, etc. doesn't grab you as something you want, then I'd say the S0 is sufficient for evaluating the activity tracking functions of the watch.
Now, if you are trying to use an app, such as podcast, on the S0, and finding it unusable, you might want to try it on a later generation watch, because it's likely performance would have improved.
But basically, the watch is best as an activity tracker. That's the only reason I can think of that makes the watch a must-have, rather than a nice-to-have. For all other functions, you can do them just with the iPhone, although with some functions/situations, they can be more convenient on the watch. Like the timer.

Notifications and trip directions are the other things that come to mind as being more convenient on the watch in some circumstances.
I do suspect a lot of people have the Apple Watch, even though they don't use its activity tracking functions, because they like having a watch, and they enjoy its fashion aspects, like being able to swap bands and faces. But you seem to be asking what makes the watch useful to people -- and the only thing I find really useful is the activity tracking. And the timer.
