I also just moved from a stainless S4 to the Ultra and love it! As was mentioned, the extra battery life alone is a huge upgrade and makes it worth it. I just put it on the charger in the morning when I'm in the bathroom and when I shower at night, and it stays on my wrist at all other times. With that routine, the most the battery ever gets down to is ~70-80%.
Given the battery life, I love being able to track my heart rate all day and use the new sleep tracking without having to plan a specific charging time and wait for it to fully charge up.
The other one for me is the durability of it. I'm on my mountain bike 4-6 days per week and like not having to worry so much about breaking it in a crash. I do still use my Garmin as my primary recording device, as the AWU unfortunately doesn't support ANT+ to connect to all my bike sensors (power/cadence, speed), but I am able to use the AWU as an auxiliary HR sensor (using the Echo app) to connect to my Garmin, since I don't like chest straps.
Additionally, although I typically stay disconnected from my phone and keep it on Do Not Disturb while riding, I like having the ability to have quick access to my phone, all from my wrist.
For shorter ~1-1.5 hour rides, I've also been about to head out without my phone and just had the watch on cellular. I can unlock my car without the keys, so it's nice to be able to go ride with nothing at all in my pockets.
I also ski and occasionally hike, and I've found that the WorkOutDoors app lets the AWU do everything a Garmin will do, minus the lack of ANT+ connectivity. I think that's pretty much just a cyclist problem though, as I don't think any other sports use ANT+ sensors. The AWU with WOD will be great once ski season is here and for hiking.
To answer the original question - I definitely think it's worth it for my needs. I originally debated between a Garmin Fenix 7X and the AWU - bike computer on wrist with some smart watch capability vs a more dedicated, true smart watch with a bit of ruggedness. Given that I already have a Garmin head unit for my bike, I picked up the AWU thinking I'd just test it out and return if I didn't like it, but I was pretty much sold within the first day.