X-Chat if you refuse to use mIRC, feel the need to pretend you are 1337/unique, or are an admin and run a crapload of scripts that you yourself wrote in a way that you can't port them to mIRC.
That first reason is the only reason I switched to XChat. I refuse to look at that bloody mIRC nag screen with some dodgy looking guy's picture on it (I know who it is, so don't lecture me on the subject..)