Scripts will still work as long as they have the standard #!/bin/bash at the top.

Aliases - in .bashrc - unfortunately will need to be replaced by Fish functions; defined in the "~/.config/fish/config.fish" file by default.

The only real thing that keeps tripping me up with Fish is that it doesn't support shortcuts like "!$ ". I keep on typing that in, but due to the nature of Fish, it can't (or won't) recognise it. But the excellent history functionality does make up for this a fair bit.

Having said that, Fish is by far the best shell I've tried. I gave zsh a go for about half a year and got fed up with the autocompletion and general slowness. As a shell, Fish just behaves quickly and fairly naturally, with few exceptions (!$ ...).

I highly recommend it as an improvement over bash.

Yes -- fish is not bash, so anything defined in your .bash_profile etc. will break, though there's usually an easy alternative way to do it in fish. Scripts should be fine though, since they usually explicitly call bash when you run them, so will still run using bash.

I switched to fish about six months ago and have been very happy with it. I spend a lot of my working day in the command line, though not doing anything exotic, and find it very pleasant to use.

Fish is not a POSIX shell and does not try to be one. In contrast, bash, zsh, and others are supersets of the original sh and backwards-compatible. Fish is not.

For example the exit code of the previous command, instead of POSIX $?, you can get it as $status.

Look out for errors with third-party applications that shell out and expect to find bash or something compatible. I had to explicitly set shell=/bin/bash in .vimrc before any plugin initialization in order to avoid breakage after changing my login shell to fish.

I've found that leaving your login shell as bash but making fish the default executable for any terminal emulators you use will pretty much prevent any breakage.