I tried using oh-my-zsh, but all of a sudden bringing up a new terminal (on OS X) went from taking ~1 second to ~5 seconds. Totally killed my interest in zsh.
I'm sure there's a way to make it all work nicely, but bash has been doing just fine for me, I don't feel the need to figure out an entirely new configuration system for a tool that ultimately has little new utility in my workflow.
There is a cleaner 'version' (==fork) of oh-my-zsh that fixed the long startup time for me: https://github.com/sorin-ionescu/prezto. With nice, fast setup (overwrites your .zshrc file!) and lots of documentation.