You can configure anything in Irssi using variables. For example, you can change the shortcuts to make Irssi behave differently when you use the Emacs-like shortcuts, you can add servers and channels which you want to auto-connect when Irssi starts, you can make Irssi log your conversations using date specifiers and any path for the logs, and it also supports themes.
Typical Irssi running inside Konsole
The Perl scripting support is one of the strong features Irssi provides: you can basically do anything, like now playing scripts, system info or weather scripts, IRC robots (though I recommend a standalone C bot for trivia games or other helping bots).
The official website is very abundant in documentation, providing help for anything related to Irssi. It's true, if you come from a GUI client like XChat or Konversation, Irssi will be a little tough to learn when you'll start using it, but once you get to know the basics, it will perform very well.
Updated: Jun 18, 2008 (Created: Jun 18, 2008)