I use Colloquy. It is a very "mac like" OS X client. People have knocked it in the past for being slow but it have vastly improved in performance on my computer as of late. For the "mac experience" nothing else comes close.
Here are some Cons about it thought:
1. No dcc chat.(it does have dcc transfers though)
2. No Fservers(I love these and this could be done through a third party but requires dcc chat )