Personally, I spend so much time in the shell that it's definitely worth spending some effort to increase my ability to use it productively. I switched from tcsh to bash about a year ago after decades of using tcsh. Switching to bash was a mistake; I probably should have switched to zsh instead. Bash is broken as an interactive shell in more ways than I can enumerate; it is a constant source of annoyance and frustration. I have no great love for tcsh, but it is far better as an interactive shell, despite the fact that it truly sucks for programming.

Re what comes with the OS: I haven't logged into a Unix/Linux box in a decade that hasn't had zsh on it.