I can't (easily and reliably) get OS X to run on my non-Mac computer while Debian runs like a champion, this is what's missing from OS X :) Otherwise I would like to try it seriously as my main OS if I get a chance in the future. Heck, I even used Windows 7/8 for a few years and even they get good enough after installing Cygwin, so I can't think of any showstoppers that would make me dislike OS X. (Well, lack of the vast apt repository
may be a problem, but they say very good things about homebrew and MacPorts, so this wouldn't be an issue I guess.)
I was a heavy OneNote user in Windows and accumulated a lot of stuff in it, so after coming back to Debian, I desperately tried running it in Wine and also tried the Web version but unfortunately they didn't cut it. Currently I use CherryTree[0] and it gets the job done but it's very far away from the OneNote experience.
[0] http://www.giuspen.com/cherrytree/