So, IRC clients.

[Mystic]

xChat. I use Trillian for IM, but I don't like using it for IRC.

[Lotos]

Pidgin, but mIRC for my laptop.

[bluaki]

I used Pidgin before. It's nice if you're only in a couple of IRC rooms at most, though even with plugins like IRC More it lacks some simple features like: merging Global/etc. notices to a single tab rather than being bombarded with several junk windows on login, badge icon for SOP, server-defined commands support like /cs, and formatted text.

After I started staying in not only numerous rooms but also multiple servers, I switched to Xchat and love the change. It's much easier to manage many conversations with it and the tree, and it also has great customization of visuals and simple user-defined commands. Seems like the perfect dedicated IRC client to me. The only thing I miss from Pidgin is the great variety of nick colors, which no other program out there has.

I personally think Trillian is ugly for any purpose (though at least better than Empathy) and that there are tons of things wrong with mIRC's interface and other details of design. Even aside from being non-free, various issues mIRC has make me regret not restarting cookies from scratch as its own program long ago; a big chunk of my code is visual modifications just so I don't have to deal with a big mess when I try looking at the mIRC window. I haven't used Chatzilla at all really, but it doesn't look pretty to me.

I use Trillian. It's not that great, but it keeps a basic and nice layout without putting ugly poop everywhere like XChat and Pidgin.

For Xchat: Though the defaults aren't so great, you can pretty much remove or rearrange any extra GUI elements in Xchat like channel mode buttons etc.
For Pidgin: What about its interface is so different from Trillian? The only major thing I know of is that Pidgin's style actually fits in with the rest of your programs while Trillian uses its own. The conversation window is even organized exactly the same in both of them.

[…]

I used Pidgin before. It's nice if you're only in a couple of IRC rooms at most,

That's what I hated about Pidgin, and Chatzilla too if you had enough of them open, because I couldn't see the room names anymore. Also for Chatzilla if Firefox crashed.