Personally, I prefer KVirc. http://www.kvirc.net/ Open source, Win32/Unix, and some very very nice features. I'd reccomend checking it out, mIRC is nice and good, but KV surpasses it greatly, and it's free.
Howdy folks! We've now got an IRC channel, Feel free to come by, hang out, and chat amongst yourselves. We're at irc.darkmyst.org on the channel #nLite Stop by sometime!