The primary reason I chose over zsh/bash was that autocompletion just works. If you install it, I highly recommend oh-my-fish as well, which allows you to add packages for look/functionality:
https://github.com/oh-my-fish/oh-my-fish
Another vote for fish. I would say another benefit is that it has a reasonably great out of the box experience with little customization needed to get a lot of its benefits, eg awesome autocomplete.
The downsides have been mentioned, namely the lack of compatibility with shell scripts not written for it. In the same vein, until somewhat recently, operators like && did not work for it.
It’s still my daily shell, and I really only use something like bash when having to remote into a production server.
I really enjoyed using fish for a short while, but soon I learned that it was not POSIX compliant. This was a deal breaker for me since my colleagues would often share scripts that wouldn't just work for me. I use zsh nowadays.
I love zsh, but it isn't without its own strange quirks that make compatibility exciting at times. In a thread from a fortnight ago¹, RANDOM is handled very differently for example. Try `echo $(echo $RANDOM) $(echo $RANDOM)` in zsh and bash.
I'm unsure whether minor differences that catch you out occasionally are worse than huge differences that you always have to think about. For me zsh still wins ;)
Without the same fluency in bashism from using it for day-to-day activities, the colleague provided script is less readable and less editable for the fish-fluent, bash-inarticulate. Any fish-specific scripts are also of low-value to colleagues.
It's not that it's impossible to be a shell polyglot, but GP deemed it easier to learn !fish.
I tried fish for a while and loved the history recommendation mechanism, but the 'vi mode' emulation had enough annoyances that I went back to zsh. From the bugs filed on it, it seems like the vi fixes weren't really possible.
I did install a similar history recommender in zsh and have been pretty happy with that.
The primary reason I chose over zsh/bash was that autocompletion just works. If you install it, I highly recommend oh-my-fish as well, which allows you to add packages for look/functionality: https://github.com/oh-my-fish/oh-my-fish
If you're a zsh user, they have a similar framework: oh-my-zsh: https://github.com/ohmyzsh/ohmyzsh