Rated 4 out of 5 stars
This IRC client has one of the best interfaces I've seen. It's customizable with its CSS, and the light motif looks great, especially with adjustments to the fonts. Adding tabs to auto-join a server and/or channel on startup is a breeze, and chat logging is easy to set up (and outputs to human readable plain text files). Colors and highlighting make each type of activity very easy to distinguish, and the flashing taskbar button is a great way to keep alert about mentions of your name or any selected keywords for notification.
My only two gripes would be that a.) You can't restart Firefox without restarting Chatzilla, which means that if you want to use Chatzilla uninterrupted, your Firefox memory usage will slowly build up; and b.) the flashing orange taskbar button in Windows is equivalent to a bouncing Firefox icon in the Mac OS X dock, which is a bit less helpful (as it's not immediately clear whether it's about Chatzilla or some other Firefox process grabbing attention). It'd be wonderful if Chatzilla could somehow work with the Growl application or somehow have its own Chatzilla icon representation on the dock.