Also - once you master it - it doesn't really matter what system you are coming from - the interaction is the same. Windows, OS X, Linux, Heck - OpenBSD console - the interaction is the same.
I wouldn't be surprised if my Circa 1980 IBM 3101 Terminal is mostly supported by Tmux. (maybe a missing Meta Key, but that isn't used that much by Tmux)
I think, for me, is that I'm also an emacs user. I never liked using the shell from emacs (I've seen people do it, it always felt clunky to me). So it's a similar configuration, but for more than just the things I use emacs for and running any tools/editors (I still use vim from time to time as well). When trying to understand some new (to me) program with a decent man/info page I can open up two panes side-by-side and review the documentation while playing around with the tool.
Another benefit, to me, is that I'm not always on the same OS (locally). So I use it a lot remotely, but it also means I'm not trying to use (on linux and OS X) two different terminal programs with different behaviors. Having a consistent environment (like I've also done with emacs, zsh, and to a lesser extent vim), makes switching between systems a breeze.
Creating new windows is the same command, switching between them is. And as others have mentioned, detaching processes without thinking about it beforehand (like you'd have to do if you start them with nohup/dtach).
Granted, the Xzibit-ness of it all grinds my wasted CPU cycle gears sometimes. Strangely enough, I haven't seen a setup where e.g. the X11 frontend just connects to a remote detachment & management daemon over ssh, so only one program is emulating ancient terminal hardware...
I could also imagine using one window manager on remote systems and locally is just easier to remember.