Why do I want to use tmux instead of screen?

If you're a Linux user, you probably don't. Let's not fight.

But, if you like good, clean code and prefer the BSD license, tmux is for you.

As of OpenBSD 4.5, tmux is in base.  It builds and works on Solaris, Linux,
and OS X.

The developer, Nicholas Marriott (nicm@openbsd.org), is very approachable,
fixes bugs quickly, and is open to new features (where is your diff?)

Also, now that tmux is developed in the OpenBSD tree, it will get many
more eyes on it and should be even better.