For me is it's awesome, out of the box, syntax highlighting, auto-suggestions/auto-complete and the up arrow history function (that includes substrings).

it's zsh without having to add in all the plugins. on my NixOS systems zsh+plugins caused it to run slow. Fish just doesn't have that issue as everything is baked into it. setting up functions is also a breeze and the built in themes are great. just one install and no need to configure much if anything.

As a ux/ui nerd for me its mostly that its a very thoughtful user experience. I have been more or less good at the terminal at different points in time, but if you don't spend much time there its easy to forget every command and how to do things. And ultimately, I don't really enjoy using it as an interface. Its a blank void with little to nothing in the way of affordances-- remember the magic words and you can do anything. But the fact its only used by technical folks means often no one cares to add affordances or ease of use even when it might be possible, and when they do its from a power user perspective.

Fish feels like a much more thoughtful execution of how to interact with a command line interface, and doesnt require a ton of time spent in a terminal to achieve that better ux. Though I still want to set up a nice prompt that improves legibility and make things a bit easier still, I just haven't played with starship enough yet.

There are more things I'd wish for in a perfect world, but for me fish is a very nice step in the right direction :)

Completions, plugins, and themes built in. It's fine. I still use bash for scripting but fish for interactive terminals is nice with little effort.

zero downsides and all upsides? like, zsh with plugins is just fish out of the box. I used zsh for a long long time (and several other shells), but with the plugins you want to have you immediately hit long loading times to actually start your shell. and then when it's finally loaded it's still a worse experience than fish.

Basically every non-Windows system comes with a POSIX-compliant shell at /bin/sh. If you're using shebangs in your POSIX shell scripts like you should, using Fish as your Friendly Interactive Shell shouldn't be a problem.