It's funny to see the first 3 comments are essentially "looks good, but I'll keep using X" where X is different each time. I was just thinking that it looks good, but I'll probably keep using XChat.
IRC Clients: They may not look pretty, but there are tons of 'em.
Looks nice, real similar to mIRC but for some reason I feel I will find myself still preferring irssi. A new shiny look on an old dusty protocol doesn't give me any nostalgia.
I prefer Textual. Open source if you want to build it yourself or throw the dev's 5 bucks and get it from the mac app store.