I've been using IRC for years and years, and more years. Before I "switched" to Mac a year and a half ago, I had always used Xircon for IRC. I always liked the feel of it more than mIRC and others. Once I started using OS X, I found that X-Chat was very similar to Xircon, so I stuck with that. I had seen Colloquy around the 'net a few times, but I always ignored it, figuring I'd feel the same way about it that I felt about mIRC during my Windows days.
I finally decided to try out the nightly build of it out of pure curiosity. Well, I'm a changed man. Thanks to its extreme level of customization, I was able to get it working and looking exactly the way I wanted. Once I felt comfortable using it, I started discovering how unique of an IRC client it truly is.
From the buddy list to the alerts to the CSS/XSLT based customization, Colloquy is amazing. It's not just the best IRC application I've used, but it's one of the best applications I've seen on OS X period. That's saying a lot. To top it all off, it's free and open-source.
I can't thank the developers enough for working so hard to create Colloquy. It's amazing as is, and I can't wait to see how it continues to evolve. They'll definitely see a donation from me soon!
Make sure to grab the newest nightly build ( http://colloquy.info/downloads/nightly.zip ) and some third-party styles for it ( http://colloquy.info/community/index.php?ColloquyStyles )!