Pidgin is okay, I've used it before, but with lots of channels it becomes hard to manage, especially if you use MSN or gtalk with it as well.
IRSSI is pretty awesome under linux, as it is text based and looks pretty good with a few simple plugins. By the way, I just run one instance... it's been up... maybe ~150 days?
mIRC I've tried, but never understood or got into, and there's always a huge delay at the start.
Peter
