At the risk of starting a religious war...
zsh is a little snappier than bash and offers some neat options which are not present in bash (at least not out of the box AFAIK - e.g. spellcheck).
From my experience, zsh targets power users (and people who really don't like to type). For most people sticking with bash is probably leaving well enough alone.
From my POV it's not a religious issue, though in just a short while of using zsh I find it more productive than bash.
