I found Conversation surprisingly difficult to use. XChat does everything in one window (except for the initial pop-up). And you can use most IRC commands at any time to use XChat.
That has and always should be the first and foremost purpose of an IRC app, ugly or not. Conversation doesn't cut it for me.
there's a great little app called colloquy (http://sourceforge.net/projects/colloquy/) that's open source and written in cocoa. it has the best user interface of any irc client i've ever used, and all of the features a general user would need. it lacks many of the advanced features of x-chat and seems not to be under development anymore, but it is definitely worth trying, as it's small, fast, easy to use, and very good looking.