The Gear S was my first smartwatch, and I bought it because it had stand-alone capabilities. I never saw the point of smartphones until the gear S.
I run on trails in the woods, and with phones getting ever larger, I bought the Gear S so I could leave my phone behind and still be available, plus I needed something with GPS that would track my runs, and something that would play back music on my runs. Unfortunatley, the battery does not last me an entire run if I track the run AND stream music over bluetooth at the same time, so I only use the Gear S to track my run with Nike plus and have cell functionality. I use up about 40-50% juice per hour this way.
When it was still working, my calls and texts were forwarded from my phone that I would leave in my car to my Gear S. Samsung being Samsung pushed some update that broke this functionality for me, so I have been using my Gear S in total stand-alone mode since spring, and as long as GPS and 3G still work I'm OK with that, but of course wish it all still worked like on day 1.
A LOT of use case scenarios I read about in the past months would be working exactly the same way if people used my $70 BT headset (LG tone plus) instead of the Gear S. The fact that it alerts me when I accidentally left your phone behind and I can answer and initiate calls straight from the headset that is around my neck during the day means that I would never need and never used the Gear S for that. If that's the only reason you are considering the Gear S 2 3G, you may be better off with just this headset.
If you are active and/or outdoorsy and can imagine leaving your phone behind, the Gear S 2 3G may be for you. I am actually looking at the Urbane 2 LTE over the Gear S 2 3G as my next upgrade for three reasons:
1) I lost faith in Samsung. They delivered chargers with a design flaw that makes them break within months, but never admitted to it, and they make you send in your watch along with the charger to get it replaced. It can take up to 2 weeks to get your watch back.
They also never released the promised additional bands, their app store experience is abysmal, and no new noteworthy apps have been added since its release.
2) The round watch will display less info and seems too small when compared to the awesomeness of the Gear S screen. And I am a woman with small wrists, so there goes the argument that women prefer smaller smartwatches.
3) The Gear S 2 3G has an embedded sim card that I can't access. That makes it effectively a limited/locked device I won't even consider. I think the Urbane 2 LTE has a user accessible nano sim card.
ETA: my Gear S is always on my wrist and it still looks like on day one one on the front. It has been pawed many a times by dogs and never even got a scratch on it, which speaks to its durability, but it also has desintegrating charging contacts on the back, which along with the charger issue, speaks to its design flaws. In other words, I have a love-hate relationship with Samsung, mostly love for my watch's hardware, hate for the app ecosystem, and now that there is a new player in town, I am looking elsewhere...with a totally guilty conscience.