first mIRC's UI can be changed to basically anything you want, and scripting for it is a double edged sword. On one side you have a language developed solely for that
IRC client, which forces people to learn a new language if they care to script for it. On the other side you have the fact that so many people use the client that scripts are readily available.
Personally I use mIRC because XChat fails to provide a multi-row tab bar so I would have been stuck with either scrolling in the treebar, or scrolling in a single line tab bar. I like Xchat because it supports so many languages that when I need a script I can find someone out there was have never really touched
IRC to help me (Besides its native in linux)
As far as the macs go I don't have a mac atm, but have helped many people with
IRC and macs and from what I gathered many of the clients on the mac are limited for power users (and Xchat Aqua is missing many features).