I quite happily use the Siri Watch face on my Series 0, and have since watchOS 4 came out (except for occasions when my nieces kidnap my wrist to play with the Minnie Mouse and Toy Story watch faces). I really like that I don't have to dedicate a complication to things like the stopwatch or timer, they just show up when they're in use.
I, too, am wondering what will happen with always-on mode (my Series 5 arrives on Friday). A lot of the newer OS4 watch faces looked cool, but were beyond my reach before, it's possible I'll run one of them most of the time, but I'd still like the Siri face to be around.