On iOS you can connect and even install apps: Samsung's iOS app is not good, but it's more versatile than a basic Android Wear conduit. Samsung S Health connects for fitness tracking, and a handful of apps and watch faces can be downloaded. Just not all of them -- Spotify and Uber don't make the cut, for example.
Samsung S-Voice works... pretty well: Google's upcoming Assistant improvements to Android Wear, and Siri on Apple Watch, offer more connections to phone functions. S-Voice still works OK, though, and can do more than you'd expect (setting alarms, calling a contact, getting weather, asking what an aardvark is). It can retrieve brief entries from sources such as Wikipedia.
What's not so hot
Tizen (and its app deficit): Samsung's watch makes yet another bet on Tizen, its own software and app ecosystem. And no one else has. The amount of apps for Gear watches has trickled to a near standstill. And while there are some clever games and watch faces, and a few brand-name apps like a CNN or Bloomberg or ESPN watch face, or Uber or Spotify, these apps are so few and far between that you'll hunger for any new one just to justify your Gear purchase. There are technically thousands of Gear S3-compatible apps... but few of them are anything recognizable, or anything most people would want to use.
The interface: Last year, the Gear S2's novel spinning wheel design for navigation was eye-popping and fun. But it's also time-consuming, and I can't get to what I need as fast as I can on an Apple Watch. Too much dial-spinning, and two buttons to push instead of one, add up to a lot of wasted time fiddling around. At-a-glance info isn't always easy because most Samsung Gear watch faces are light on customization. I can't pack on shortcuts or quick readout info like on the Apple Watch -- say, to music or weather or calendar.
A more efficient design is needed. It's definitely better than Android Wear right now, but it's not another leap forward. And getting some apps to work requires a mix of Samsung apps, conduit apps and maybe even double log-ins. (Spotify required another log-in once my phone was out of reach.)
LTE model can drain the battery fast: I did get two days of battery life on average using the S3 Frontier LTE model when paired to the phone, but if I used LTE to connect to the watch away from the phone, battery life slipped away fast. Depending on what I did, my watch could drain before I got home for dinner (and my watch charger). A day on LTE standby was typical -- but not guaranteed. What's the use of a smartwatch that has such a wide range of battery endurance depending on performance?
It's water resistant, but not swim proof: IP68 means dust and dunk-friendly, but unlike the Apple Watch Series 2, you're not meant to go swimming.
Would I buy one?
Would I recommend the Samsung Gear S3 to an iPhone owner? Absolutely not. But with Android phone owners, the decision is harder. There is no perfect watch right now, just a field of inferior products. Android Wear isn't a good answer now, althoughcould provide some promise starting in the next few months. . Samsung offers some interesting options with the Gear S3, and its ability to be a full phone if you spring for the LTE model could appeal to some people.
After more than a year since the Gear S2, the S3 amounts to a lateral move. Improved hardware, but not improved software. And no matter how clever the S3's design is, it needs to be better at delivering information fast. That being said, right now, I'd prefer the S3 over any current Android Wear watch... or any other Android-connected watch. That statement will probably change sooner rather than later. But the older Gear S2 is still available, and at around $230 it's far more of an impulse purchase. It lacks a speakerphone, though. And if you care about adding phone service, the Gear S3 LTE is far better than the S2 3G.
Samsung's watch is absolutely attractive. Its looks catch the eye of people I show it to, and while it's big, it's definitely good-looking. But it's not the best info-on-my-wrist remote way to quickly check in on my life. Maybe that's a problem with all smartwatches. But it's definitely a problem with this one.