I loved my Apple watch until it couldn't send messages anymore (and neither would the iPhone. "Message failed to send" only affecting SMS)
That being said, the Galaxy Watch Active is a lot like a series 3 Watch with a smaller, round display. I think it's a nice alternative. Samsung Pay works quite the same way as Apple pay as well. I also got a Gear S3 Classic for MST payments (for places such as Kroger who have no NFC support much less tap to pay. MST simulates a card swipe)
The battery life is on par with my Series 3, with AOD off on the Samsung. Bixby works a lot like Siri (for bettor or worse!) But handwriting recognition is awful; Apple wins here. Better to stick with dictation or the t9 keyboard if you can get to it (used to work with the 'digital bezel' until last update now just vibrates but doesn't switch keyboards.)
Bands are similar. Mine has the sport band which resembles the Apple watch sport band. Wifi seems to work just as well at connecting remotely to the paired phone as an Apple Watch would.
Used to be, though, if you didn't own a Samsung phone, any text messages wouldn't be able to be responded to via the Galaxy Watch. There was an update to the OS of my Active last week that has changed that. Now messages is accessible via the Watch even though it's paired to a Motorola G7 Power.
The Galaxy Watch has a tad wonky sleep tracking. It's better than it was, which is, you'd get up to do a bio-break in the middle of the night, say you went to bed at 10PM, woke up at 3AM to pee, then wake up for real at 7AM, instead of saying correctly you slept for 9 hours, it would just count the time since you went to pee. So you'd get an incorrect sleep record of 4 hours total. Also, would make Samsung Health complain about lack of sleep after so many times of that. Lately, as of last update, it stil shows the breaks in the sleep but the total time is accurate lately.
Don't expect the durability of an Apple watch though. The Galaxy Watch is easy to ding up if you're rough. There's also no real 'premium' build. The 'classic' gives you a leather band and a fancier bezel but it's even easier to ding up the bezel since it is so out there. Screens are easy to scratch or 'hash up' in my experience. Definitely get a screen protector.
Don't let too many apps run either. After five or so, it starts getting laggy, slow or drains the battery in a few hours. Some apps take a few seconds to work. Samsung Pay is no Apple pay in speed. Apple Pay pretty much instant. Samsung pay, hold upper right button, blank screen for 3 seconds, Samsung Pay logo appears, 3 more seconds asking for PIN, 2 seconds later the Pay button lights up, you tap that and it then works. It's slow. A lot more steps than necessary IMO.
The lack of a 'radio' app kinda sucks too. If you like using your Watch on Wifi or just connected to some AirPods enjoying some 80s music without your phone near you, forget that on the Galaxy Watch. The only 'radio' app it has is Spotify, which requires the same app on the phone, and a premium subscription for any real function. There's no real station by decade the way Radio on Apple Watch is. The 'music controller' just controls the last app you had on your phone playing music. It also goes to 'sleep' showing 'unknown' or 'no song playing' which means you must get your phone out to restart any app such as Slacker or SiriusXM. Forget asking Bixby to 'play some 80s hits'. It will say 'that is not supported'.
While Bixby is great on an S8 (no rhyme intended) it is just a renamed S-Voice on the Galaxy Watch. It has a nicer UI and works but it's functionally the same as S-Voice was on a Galaxy S3 phone. It can open apps, turn volume up or down (on a watch with speaker) and set alarms. It can also control SmartThings accessories. But anything else, such as setting reminders, showing your recent apps list, closing apps, toggling Wifi or Bluetooth, it cannot do it. All that is possible with Bixby on a Galaxy S8 or up, but not on a watch. A lot of functions it should support are either 'to be in the next update' or 'not supported on Galaxy Watch.'