I started using Mac OS X last year and I've never looked back. My prospects for finding a good mud client that I didn't have to run in classic were dismal. I wanted a robust, user-configureable mud client that would run natively on my system.
I had already decided I was going to teach myself Cocoa, now I had a goal to accomplish. In all fairness though, building a mud client gave me an excellent reason to play Avatar, my favorite mud.