My models so far have been:
Series 0
Cellular Series 4
Cellular Series 8
Not spacing them out that way on purpose or anything, that just seems about how long I get out of a battery per model.
My cellular Series 4 was wonderful, and I probably could have squeezed at least part of another year out of the battery, but I decided to stop waiting around and go for it. So the main thing I like about the Series 8 is that it has a fresh battery.
BUT there are other perks from the upgrade too:
--Bigger screen is noticeable and more useful
--Always on screen is infinitely more useful than having to rotate your wrist just the right way
--I'll always appreciate health sensor updates. I'm not getting any younger.
--Connections to AirPods, cellular signal, wifi, etc. seem to happen instantly now. On my Series 4 I felt like I often had to wait or try multiple times to get my Watch to connect and stay connected to my AirPods.
--Apps and widgets open and refresh much more quickly and reliably.
I think it's worth the upgrade, especially if you find yourself putting the Series 4 on the charger earlier and earlier in the evening lately.