I started using zsh when I was a graduate student, and at the time, there was no shell to compare with it for interactive use. The great command completion support was one of those reasons. The bash completion project come up (again) in the freshmeat.net RSS feed. These folks seem to be revving their completions pretty quickly. In the meantime the zsh folks have come up with a new super spiffy completion system in zsh 4, and while there are lots of (very complete) completions, they don't seem to be revving them as fast. That and the zsh 4 system is more complicated (and capable) than the previous one (in fact I've dumped all of my old completions and am just using the new defaults). And truthfully, I'm not sure I have the time to read the manual to figure out all the completion system settings or the syntax for defining new ones. But there are all these new Mac OS X command line commands....